Latest ForceWare drivers? The nVidia drivers that ship with Vista are
abysmal.
http://www.nvidia.com/object/winvista_x86_163.69.html
The above link assumes you're in the USA. If not, click the International
Driver link on the same page.
It's also important to fully remove the old ATI drivers BEFORE changing
cards. ATI normally supply a utility to do this: it can be found under
Add/Remove Programs and if memory serves it's called 'ATI removal tool' (or
something like that).
If you're already changed the cards over without fully removing all ATI
software, you'll have to reverse the process:
Uninstall the nVidia drivers
Remove the nVidia card and put your old one back in
Run the ATI removal tool
Swap the cards back and install the latest ForceWare drivers.
I have an 8800GTS with 640MB and that has Graphics and Gaming Graphics
scores of 5.9, so I think yours should definitely be scoring more than 3.5