um it depends,
If you get a Pro version of any of the ATI cards or the stock ones, then 128mb should be good. But if you go with an SE then you'll want 256mb of ram. when you check vid card specs take a look at the ram speed and gpu speed. If you go with nvidea sme deal, if you get an LE version, get the 256mb, and if you get a regular or ultra, then 128mb should be fine.
One more thing... PRO's and ultra's are more expensive for a reason, they go faster than the LE's and the SE's.
PS: Ati's SE line and Nvidea's LE line are budget cards, so they go slow.