I don't have TS12 but going on my knowledge of how things work the dual graphic cards normally need a special driver and I don't think TS12 is mainstream enough for one to be developed. I'd go single graphics card at the moment.
Currently I've seen TS2010 use 8 cores when in Content Manager, I suspect it could use more, in Surveyor and Driver I suspect it uses one thread (core) for running the game display and one for the AI side etc. My expectation would be that they will attempt to add threads as dual processors with hyperthreading and quad processors become more mainstream.
Intel has some very good optimising compilers which will optimise code for Intel processors so I suggest that an Intel quad wouldn't be a bad choice but currently a dual core would give the same performance but be slightly cheaper.
At the high end much has been said about the benefits of SSDs and if you have money then these might give a better return in performance terms than a second graphics card.
Cheerio John