decent card for 100 bucks... that will be difficult...
two things first:
1) is it AGP or PCIe (aka PCI Express)? they are not compatible with each other so check which one goes in your system. google up some pics of AGP slots if you're not sure.
2) with both ATI (radeon) and NVIDIA (geforce) the first number is always the generation number, and the others are model numbers. example: a geforce 7800 is a 7th generation nvidia card, and the "800" means it's the high-end part. an 8400 is a low-end part of the eighth generation, much slower than the 7800. google up some tests (comparisons on anandtech or tomshardware for example) to see how a card performs.
one more thing: memory is overrated. more is always better, of course, but an 8400 with 1024MB will not even get close in performance to an 8800 with 512MB.
right now the best deal are ATI's 3800 series. they were supposed to be high-end cards but failed to perform as well as nvidia's offerings, so they have all the nice high-end features at rock-bottom prices. here are two you should consider:
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the 3870 is simply a faster version of the 3850, so if you don't have the extra $25 you can get the 3850 and overclock it a little...
there are cheaper cards like the radeon 3600-series or nvidia's 8600, but they offer much less performance for only a few dollars less since they were always intended to be mid-range cards. here is a comparison test:
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if you still have AGP the same basic rules apply, the higher the model number the better; but since AGP was phased out some time ago you will have to make do with older cards.
if you have more questions just ask.