Good then, we have a good place to start.
If you intend to use crossfire 4850s, or a 4850X2, that suggests a certain budget.
Also, crossfire 4850s are good bang for the buck on a 22", 1680x1050 monitor but would not be as good a buy on larger monitors.
A 790FX board is something you buy to run two 4870s on, for a resolution of 1900X1200 or higher. Of course it's a deluxe board and may have a feature or two other than crossfire bandwidth, but mostly people buy AMD for just the reason you say... bang for the buck.
So, you get a 940 and pair it up with the equally economical 790X board and 4GB of 1066 DDR2.
Put as much money as is left over into GPU(s) and a PSU of high quality with enough amps to power the GPU(s), because THAT is where you get all the performance for gaming.
This is why we have the sticky at the top of the forum, so we avoid all this by knowing exactly what's needed up front