Every system has a bottleneck, and it depends on the software you use more than anything else. CPU, RAM, videocard... it all depends.
In general, for gaming, your Sempron is going to have a hard time with newer titles (Doom3, Half-Life 2, etc).
If you want the most bang-for-your buck out of your current platform, you should probably:
1- Get a used Radeon 9700 for dirt cheap on ebay, or a new 6600GT. Better yet would be an X800GT, but I'm not sure if those come in AGP.
2- Get 1gb of RAM
3- When you can afford it, dump the Sempron 2200+ and get an AthlonXP 2600+. Much faster processor in real life.
Of course, after all that it probably makes more sense to get a new system... so to make it worth your while, the best bet might be to get the used 9700 PRO and run your system until you can afford to upgrade entirely. They go for as low as $45 on ebay, don't pay more than $60 though.
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<b>Geforce <font color=red>6800 Ultra</b></font color=red>
<b>AthlonXP <font color=red>~3300+</b></font color=red> <i>(Barton 2500+ o/c 412 FSB @ 2266 Mhz)</i>
<b>3dMark05: <font color=red>5,275</b>