1. Your CPU is your biggest limiting factor right now.
Find out what your motherboard is. If it isn't ancient, it can probably take anything up to an AthlonXP 2600+ (266 mhz bus)
If it's newer, it might take 333mhz or even a 200mhz bus CPUs. Then you're really in business.
Either way, buy yourself the fastest CPU you can use. If you're limited to a 266mhz bus, you can probably get a 2400+ for under $100. If you're not bus-limited, get a nice 2500+ and overclock it.
2. Your second biggest limiting factor is, of course, your videocard.
If you're really poor after the CPU, for about $30-$40 on ebay you can get a nice Geforce3 (Which are actually much better than Geforce4 MXs and most Geforece 5200's with the 64-bit bus).
If you can afford a few dollars more ($60-ish), your best bet is to get a Radeon 9550 (128-bit bus version) and overclock it. If you're lucky, you can get Radeon 9600 PRO speeds out of it, in some cases people have gotten 9600XT speeds from Radeon 9550's!
Either way, overclocking is your friend.
And if you have ~$150 after your CPU upgrade, look on ebay for a used 9700 PRO or 9800 PRO.
With a 2400+ or faster CPU and a Radeon 9550 @ 9600 PRO speeds, there's no game out there you won't be able to play nicely... you won't be able to play the most advanced titles at max detail of course, but it will be very playable at lower settings.
With the 9700 or 9800 PRO and a 2400+ CPU, it'll be even better... real sweet performance for the cash.
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<b>Radeon <font color=red>9700 PRO</b></font color=red> <i>(o/c 332/345)</i>
<b>AthlonXP <font color=red>3200+</b></font color=red> <i>(Barton 2500+ o/c 400 FSB)</i>
<b>3dMark03: <font color=red>5,354</b>