It depends on what you computer has for connection to a video card. I'm assuming the SIS650 is the onboard video.
OK, you got 2 basic kinds of connector. PCI and AGP. PCI runs at 33MHz tops and is thus very slow - but if you have no sort of AGP connector (it's a brown slot, next to the white PCI ones) then PCI is the only choice. The fastest PCI card so far as I know is the GEFORCE 4 MX.
However, the GEFORCE4 MX is currently regarded as pretty slow, so if you DO have an AGP slot - don't get one!
Next, how fast is your AGP? 1X, 2X, 4X or 8X? So long as it's 2X or faster (very likely given you have a P4) then your best choices are the Radeon 9700 or the NVidia GEFORECE 4 FX. Although designed for 8X AGP they should work at 2X. And they are both out of your price range (I THINK).
So next best choice would be GEFORCE 4 TI. It too can go at 8X but is somewhat cheaper (and a lot slower) than either of these.
The FASTEST one is the Radeon 9700. The Cheapest is the TI (just about within your price range).
It pays to shop around, so maybe you can get a Radeon if you look hard enough - the 9800 is out now.
Oh yeah, one other point - GFX cards come in either 64MB or 128MB sizes. The bigger the better - but if you have a small monitor 15-17 inch then a 64MB should do if you are prepared to run games at lower res (800 X 600 pixels say, rather than 1024 X 768 or higher resolutions).
Hope this helps - happy hunting.