Something cheap like this... look at the customer reviews... one guys says he gets a 5.4-5.9 in Vista 3D with it..
Not to shabby, for reference my OnBoard Geforce 6150 gets an even 3.0 ;)
And remember not only is the dedicated video card faster for 3d graphics, but also gives you back your system memory... ;)
they have Tons of cards over there at newegg.com you just really have to get what you need for the price you want to spend... I just grabbed one..