Ok here is what I came up with, and in my opinion this would be a very healthy gaming system.
I am still sticking with the 7N400Pro2 motherboard, sure there are $50 motherboards out there that would work, but this board has the killer sound card, IDE Raid, SATA Raid, Gigabyte Networking, like I said it goes on and on, it really is a killer board in my opinion.
For a processor I think I would stick to the AMD XP 2800+, the retail package can be picked up for $121. You could get the AMD 3000+ with the 400 FSB for $140 (Then add $10 for heatsink), but I really do't see the big jump in performance for $30 dollars. Also note that I have a XP 2800+ and a 64-bit 3000+ chip, and the 2800 XP chip is faster in every benchmark I have run. I would stick with the 2800 XP, it as the best cost/performance ratio.
I found this 430 watt case:
http://www.newegg.com/app/viewproductdesc.asp?description=11-138-033&DEPA=1
You can get a more plain case for about $25 with 400 watt power supply, but to me the cool looking case is worth a little extra.
For RAM, on an Windows XP machine, 512 MB is good even for most gaming, more will be needed in real heavy LAN multiplayers. But to start I would just get one 512 MB chip, later if you want more you could buy another, and with the motherboard above, you would then be running dual channel. Don't go cheap on RAM, cheap RAM can cause all sorts of un-explainable errors. I found Muskin 512 MB PC3200 400 mhz with a cas latency of 2.5 for $77.
You can get a Western Digital 80 gig hard drive in IDE form (7200 RPM, 8MB cache) for $60. The SATA version which is mariginally faster, but a little more difficult to setup can be had for $70 for the same setup.
So in total:
Gigabyte 7n400Pro2 ... $100
2800+ XP Retail ...... $121
430 Watt Case ......... $42
PC3200 512 MB ......... $77
WD 80 gig ............. $60
----------------------------
Total ................ $400
This is right at your $400 limit, but is pretty much a complete system. Also you should note that the giga-byte does not come with on-board video, so you will need an video card. Your best bet if you want to stay on a budget is to get on eBay and pick up a GeForce4 TI-4600 or even a TI-4400 with 128 MB Ram. The TI-4600 can be had for $50-60, I have seen the TI-4400 for $30-40 on eBay. These cards are a little old, but if you do some research you will find these are still good cards, better than most new ones. The TI-4400 is just marginally slower than the TI-4600, but I think the price makes it a better choice. These cards are diffantly better than stuff like the 5200 Geforce4 (another budget card). I still use a TI-4600 card in my gaming machine, and so far run every game I have in 1280x1024 with all graphics at full (This on all current games out so far, still waiting to see about Doom3 though).
All these prices are from NewEgg.com. I have ordered a lot of stuff from them in the past and feel they are reputable, as I have never had a problem. Some places might offer things a little cheaper, but I would rather spend a couple more dollars and have peace of mind.
Good luck, any questions feel free to ask.
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