I had a GeForce 8400 GS with 512MB memory before. It was pretty nice, considering it only cost about £30.
EDIT: Btw, you can get a Radeon HD5770 which is slightly faster for the same price, and AFAIK ATI's Linux drivers are better than nVidia's.
EDIT: Btw, you can get a Radeon HD5770 which is slightly faster for the same price, and AFAIK ATI's Linux drivers are better than nVidia's.