I had a 64 Mb graphics card in my home built XP Home machine driving a wide screen 19" LCD monitor, 1440 X 900 resolution. I upgraded to a 256 Mb graphics card. I've seen no difference. Even playing Doom III and Tomb Raider. I wasn't impressed.
It's your money.
And as an after thought, Vista is still buggy with most hardware.
I Bought a new computer. Its A emachine t3604 here the system specs http://shopping.msn.com/specs/shp/?itemId=712024712. Will it work with any graphic card? SOmething under 100. It has window vista home basic.