<waiting for Alvin to post about OpenGL 3.x>
Anyways, for vid editing for non pro work, you can get a way with pretty much any card. As far as Sony Vegas is concerned, it doesn't matter much if you have GDDR3 vs GDDR5 or the amount of VRAM, hell I still have a PC doing light Vegas work with a Q6600 OCed with a softmodded 8400GS, it still works fine for small (~120 minutes or less of DVD quality vid) edits,etc.
As for you, I recommend you just get a cheap 4350,8400GS,etc and be done with it. Spend the saved money on more HDD space or more RAM (assuming x64 OS).