Okay got it.
Well, you have an almost 20 year old IBM system. To help you understand how old it is, IBM was bought out by Lenovo so many years ago that the IBM name doesn't appear on any personal computer for almost a decade now. Your system was deemed End of Life by IBM on 12-31-2009.
Now that all being said, IBM made extremely excellent systems back in this era as I still have some of them. My dad's system is an IBM 8189 and is still working today. Not only that, IBM's documentation for these systems was top notch even by today's standards.
So let's get started. First off, your exact system (8175-KQU) does not show up in the hardware service manual here:
http://ps-2.kev009.com/pccbbs/thinkcentre_pdf/19r1295.pdf
This means that it was a custom system that won't match any of the readily made configurations at the time, so we will need you to go into your bios and post what the system summary says for the RAM, CPU, GPU/video card (if any) and storage. This will let us know exactly what you have and give specific recommendations.
But even without that information, going through the parts list at the end of the service manual it is clear that 3.4Ghz and 3.2Ghz 800Mhz fsb Pentium processors with hyperthreading were options. Depending on what you currently have, this may be a significant boost. IBM is usually quite good about processor support so more than likely a bios update won't be needed. A quick ebay search shows that these processors are about $30. As IBM used 3 different heatsink/fan combinations for the cpu, you may have to upgrade what you have, or do s simple modification to make the fan run at 100% all the time. (I have done this successfully with Dell systems running 95w processors on 65w heatsinks.) The drawback of a 100% fan is that will create noise, but that's the tradeoff.
Luckily, it looks like you have the tower case, which is nice and roomy for installing upgrades.
You have 4 ram sockets, each which can take a 1GB DDR-400 module. These are still readily available from manufacturers and used on ebay. This upgrade is about $20-$25 used for 4x modules. With all 4 memory sockets filled, you will have the maximum ram (4gb) running at the maximum speed (400Mhz dual channel). This is a hard limit and there will be no way to exceed this. Furthermore, not all the ram will be available to the operating system due to the on-board video card eating some of that ram.
Which brings us to the next upgrade--video card. I have Radeon 9200/9600 agp cards in my mom and dad's 8189 systems and it provides a significant boost over stock. These cards vary in price and quality from $10-$50 on ebay, and back in this era there was a lot of performance differences in cards so you would need to research these using something like the techpowerup gpu database. For example, you can see the specs of the 9200se compared to the 9600pro in these links.
https://www.techpowerup.com/gpu-specs/radeon-9200-se.c48
ATI RV350, 400 MHz, 4 Pixel Shaders, 2 Vertex Shaders, 4 TMUs, 4 ROPs, 128 MB DDR, 300 MHz, 128 bit
www.techpowerup.com
Look at the memory and pixel rates as well as the power usage to compare the performance and power requirements.
AGP standards were a bit all over the place as well so you would need to also check the 'notches' on the physical AGP slot to make sure what you get will work. A card that does not fit will not work, and is made this way to protect from the wrong voltage card from being inserted--do not try to get around this or you will damage your system permanently!
That brings us to the last item--storage. Luckily, these boards are extremely versatile in that they can support IDE and SATA drives. You probably have an IDE drive as that's what these came with stock, but moving to a SATA SSD will definitely help. And it doesn't have to be the fastest one available either as the system will never be able to transfer the 500MB/s the fastest models can do. A brand new ssd is about the same price as a used one, with 500GB units available for around $50 or so. With the way prices are crashing on these and people are upgrading, you may even be able to get a secondhand 128GB one for free from someone generous.
The final thing to keep in mind is how to use this system even when maxed out. It was designed for Windows XP, and ideally if you are not using it for regularl Internet usage, I would use XP on it as it flies in the maxed out configuration. I would couple the XP installation with Microsoft's SteadyState, which will allow you to lock the system down completely which will make it virtually impervious to any type of malware, new or old--you simply reboot and the system returns to the exact state in which you locked it down. I use this for my dad's system and it works great. It also keeps him from damaging the installation by messing with anything.
As far as applications for XP, I would simply use portable programs like open/libre office and others running on a separate partition on the ssd that is not locked down by steadystate. You can even run portable firefox esr for web browsing as it is still relatively modern and runs on xp. However, I would not expect to do anything on this system that even a $100 chromebook can do--it will be far too slow to do anything of that nature.
To give you some idea, the website speed-battle.com is a good way to get a rough idea when comparing systems and my mom's p4 rating is 426 while a q6600 Dell 745 USFF system is 926 under Firefox ESR. This test also varies wildly between browsers, and even each run (you have to refresh a couple of times to get an idea of the real rating). The same test on chrome 45 is 303 and 530, respectively. (To best honest, I expected this rating to be much lower as the cheapest brand new laptops at walmart hit 300 on the same test, but it is a different different browser (edge) so it's not an accurate comparison.)
A system like this is perfectly capable of viewing pdfs, opening office documents, and checking email, even writing a book--as these are all the tasks my dad does on his system and it is quite snappy. His systems specs are P4 3Ghz and 2GB of ram with a radeon (can't remember which one atm), and the stock ide hard drive (yes, still running!).
The beauty of the system you have is that it was designed like the computers of the past--made to be reliable and upgradeable (for what was very upgradeable back then) with zero planned obsolescence. And if it suits your needs, you can simply keep running it and replacing parts as they wear out for as long as spares are available. Unfortunately, when things do fail, you will need to dig to find the exact IBM FRU as today's parts will not work for things like the motherboard. (I have another 8189 with a failed motherboard so I know this problem too.)
I hope this helps and feel free to post back if you need any more help.
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