It's not exactly a video card, but on a Xbox you can solder an additional 64mb of RAM on the board and get it to run with 128mb. Since the board is more intelligent than a video card, it detects it with no problem. With that consideration, you should be able to solder additional RAM onto a video card, though you may need to reflash it so the card knows it's there.

If you have the time and a couple spare video cards to test this on, give it a shot. I'd make sure I have the ROM for the video card, as well as one from the same line with more RAM. You never know until you try.
MacBook 2GHz Intel Core 2 Duo w/ 2GB DDR2 RAM & 120GB SATA 5400RPM HDD
Canon Rebel XTI
Google Cr-48 Beta Laptop