Zbytek stroje je jaký? Jestli tam máš ještě AGP slot, tak se na novou grafiku vykašli a upgraduj to celé. S těmi Vistími drivery to není tak jednoznačné - AMD jen malinko upozorňuje:

Note: AMD’s DirectX 9 ATI Radeon graphics accelerators are not officially supported under Windows 7. If the user chooses to, they can install the ATI Catalyst Windows Vista graphics driver under Windows 7. Please be aware that none of the new Windows 7 graphics driver (WDDM 1.1) features are supported (as the Windows Vista level graphics driver is limited to WDDM 1.0 level support).