I have a few of these older Dells still working as well.
Your ability to add a gpu will be very limited due to the motherboard and slot layout. Your only options will be shorter, single slot cards that do not require external power. Luckily, there are several older cards that work well in this capacity as well as several newer ones.
But before that, you may want to consider a cpu upgrade as moving to the very inexpensive e8500 or e8600 will almost literally double your performance. You can also use a q8300 or q8400 quad core which will make a difference if you're running a lot of applications:
https://www.cpubenchmark.net/compar...8600-vs-Intel-Core2-Quad-Q8400/937vs957vs1041
Personally, I like the e8600 is the best as it flies--it feels even faster than a q9650 which I have installed in other systems.
Next is the memory. If you already have 2x modules, you will need 2x new ones. 2x 2gb ddr2 for a total of 4gb. This is the max under xp and is the max for the board.
However, if you ever want to move to a 64-bit operating system and keep your cpu, you can actually swap in an Optiplex 380 motherboard which uses ddr3 vs ddr2 and can take up to 2x 4gb ddr3 for a total of 8gb. It will also be able to use your current case, connectors, power supply, and cpu so the only investment would be the motherboard and ram. The 380 motherboard also has no limits on cpu and can use any cpu in the lga775 family so you woudn't be limited to the q8xxx series like with the 330.
However after owning all these systems and running various operating systems on them--these systems are best with what was in their era--xp. But running xp today if it is connected to the Internet is a bit of a risk and I would recommend using the free steadystate addon to lock down your xp and make it impervious to any infections of any sort. Another benefit is because steadstate can redirect all disk access to memory (which is deleted after a reboot), you don't need an ssd to get immediate performance gains. I've done this with my xp systems and it coupled with portable firefox esr, they are still able to do basic tasks quite well.
And similarly, the fastest hardware I have found is the e8600 and just 4gb of ram. A gpu will make a difference as well, but you will need to make sure you have proper driver support. My 330 has a 380 motherboard in it with a xeon x3360, 4gb of ddr3, and a 9500gt video card and is quite snappy even without an ssd.
Good luck and feel free to ask any more questions.
I love seeing these older systems live on.