I've just installed the Chaintech GeForce 7300GS (GSV73GS). This card has 256MB onboard memory and can use another 256MB from the system.
I now get a 3.0 Vista rating for this system instead of a 1.0 and I get the aero effects now.
This was a reasonably cheap card that comes with a low profile bracket but the bracket only supports the dvi & svideo ports. The VGA port cable is not compatible with the system one. I plan to use a DVI to VGA adapter. For now I just took out a slot protector and have the vga cable hang out through there. The vga ribbon cable supplied with the card is a little awkward to use as it has a hard black thing in the middle making it less flexible.
I downloaded the latest drivers from nvidia and they worked well though I had to get Vista to re-rate the performance for the system before Aero came on.
Basically I'm happy with this, though I think the mediacenter is a little noisier. Could just be because the back isn't sealed right now with this vga cable. It should be better once I switch to the dvi to vga adapter and can put the empty card slot plate back in place.