What video card will fit my computer?

I just bought a new game for my computer called Oblivion, and it works- but on some parts of the game it is a really choppy frame rate. I did the "Can You Run It" test on my computer before I bought it, and I exceeded the minimum specs. I believe I just need a better vid card, but I need to know which one I should buy that fits and won't be to hard on my computer. (I prefer nvidia)



Here are my computers model/specs:



emachines W3644



AMD Sempron Processor LE-1250

2.21GHz

1.9GB of ram.

Microsoft XP Home Edition

nvidea Geforce 6100 nForce 405 512mb

57.9gb free space.



I've had this computer for a little over 2 years, should I just set money aside for a new computer, or will buying a better video card fix this problem?

I'm a casual gamer, nothing hardcore here.



I'd rather not invest more than $100 in a video card.



If you have an exact vcard model for me, please link it if you can!



Thanks in advance!|||A geforce 220 should be around the $100 mark and it's probably the best bang for your buck.