Unless you plan on running 3D accel PC games, the memory shouldn't be an issue. More memory on the card will just let you run a higher color depth at larger resolutions. Since most standard TVs only support 640x480 or 800x600, you can get 32-bit color at 800x600 with 8MB of RAM on the card (My math might be off, but I think you only need 2MB to support 800x600x32). Since most cards now come with 16MB as a minimum, just make sure the card supports S-Video output, and any hardware stretching features you want to see.
I would highly recommend ATI as the way to go, since Matrox, S3(3Dfx), and nVidia have always lagged behind them in TV output, and it's been a long time since 2D performance was measured on a video card.