I would say that varies based on the types of available slots in your machine, and the amount of $ you want to spend.
I used to have an ati X300 with 128MB physical ram. Cheap card, game played well, p4 3.0ghz and 1GB RAM may have helped.
Now I have an nvidia quadro fx 4500 w/512MB of ram. Expensive card, much farther draw distances if you tweak user opts, difficult to justify the cost just to play the Matrix.
Without knowing more about your machine (cpu, ram, current graphics cards, available types of slots; ex: AGP, PCI-E) it's tough to make a suggestion on a model. In general I'd suggest 256MB cards are cheap enough these days. Avoid all cards labeled with "Turbo Cache". Turbo Cache translates into using system memory to supplement a low amount of memory physically installed on the card. This has a negative impact on performance in my opinion.
Good Luck