I'm currently mb'ing 5 shaman on one computer, 2 monitors, an ATI 2600 (512mb) and 2 gigs and do fine until I enter a city. Out in the wild PVE'ing I don't get any problems what so ever. I am going and getting 2 more gigs today (memory is absurdly cheap right now for some reason) and possibly a better vid card. I think if you have a good enough video card, you should be ok with 2 gigs.

Siaea, Seiae, Sieae, Seaie, Saiea, level 32 shaman, Horde Bonechewer