What is your current setup? ie processor, ram, hd (size and rpm speed), cd rom read speed. Upgrading your machine will likely entail buying the floowing:
1)A new processor. unless the one you have now is 800 mhz or better.
2)A new motherboard. if the one you have now doesnt support agp.. well you need a new one to play games.
3)A video card. an agp video card blows away a pci.
4)RAM. you might need some ram also. 256 mb will keep you running fast assuming you dont have a bottleneck somewhere else (like a pci video card)
for a gaming machine I would recommend buying a new motherboard. Depending on what you have in it now (AMD or Intel, pc 100 or pc 133 RAM) I would recommend the tyan trinity 2390 for an amd (socket a support), for intel get a farily new one with an agp slot. I'm not really an intel person, so i dont have any suggested models, but I would get one that supports 4x AGP and pc 133 SDRAM, RIMM is too pricey atm to be worth it.
You wont see any really good PCI Video Cards in comparison to AGP. For AGP cards, depending on what you need to do, I would recommend the 32 MB All-in-wonder based on Radeon if you need the alternate outputs/inputs. If not, the basic Radeon card should be sufficient. it will run you about $160.
If you are strapped for cash, I guess I would recommend any 32 mb card with full throughput (not MX) and an MX only if you must (they run about 60-70$). just remember you get the performance you pay for.
hope I helped some.
-Nick