I run BTS Huge Maps + 18 civs just fine w/ 2 Gigs of Ram and a 256Meg Nvidia 7600GT
Prior to that, was playing Normal/Large maps in Civ & Warlords with 1 Gig of Ram and a 128MB ATI Radeo 9800 Pro.
I imagine the 36 or 40+ Civ and larger than Huge maps, might need more resources than I could throw at it. But I don't have much interest in that scope of the game atm.
If I take a gander at my Task Manager, BTS is using 650MB of System Memory.
The problem sometimes arises when I try and reload a save (while already in game) will get memory exceptions. Likely it briefly is using 650+ MB of ram, then trying to load the save into memory before fully releasing that 650, so my system crops up memory exception.
If I just exit out of the game, restart BTS and reload the save in question all is fine.
Guess I've rambled a bit ;-)
I think the answer is probably 2Gigs of ram, if you like the larger maps.
You likely wouldn't see much gain with a 512MB vid card.
Also when I researched Graphic Cards last year, the performance increase between 256 and 512 was very slight, ~10% or so.