SIS741 is most likely a built-in graphics chip. Not much around for AGP these days, as I only know NVidia numbers I would suggest a Geforce 7 card of some description, though you need to be sure your power supply is up to the task, 350watts minimum. For some cards, the amperage available on the 12 volt rail is important too - you can read the details on the side of your power supply.
7600GS range are fairly capable, it depends on your other hardware as to what element of your setup becomes the bottleneck, if your PC is lowish spec then a powerful graphics card won't perform at its best.
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