Let me assume that you want gaming first.
Then pick your graphics card/s first.
At 1680 x 1050 resolution, consider the 5750 for exceptional performance.
At 1920 x 1600 resolution, consider the 5850 for exceptional performance.
At 2560 x 1600 you need a very strong card like the 5970.
Do not plan on multiple cards if you do not have to. They are needed only for 2560 x 1600... maybe.
Multiple cards need stronber PSU's, more expensive motherboards and better case cooling.
Some games will not scale well with multi gpu's.
For future upgrades, look to even stronger single cards to become available. At that time, plan to sell your old card and buy the new.
What cpu does it take to drive these cards to their potential?
Perhaps less than you think.
Unless your game is a very cpu intensive, multi-core enabled game like FSX or supreme commander, then a duo clocked at 3.0 will be enough.
Most games do not depend on more than two cores for adequate performance. Game developers will not sell many games that do. Faster cpu's do help, but you start getting diminishing returns.
With a cpu budget of $300, you can get a i7-860 or i7-920. The cpu performance of the 860 is a bit better, but the 920 is easier to overclock to higher numbers.
With a budget of $200, you can get a i5-750, or the new 32nm i5-661 dual core. The 750 has 4 cores/4threads, the 661 has 2 cores/4 threads and is clocked considerably higher.
Any will do the usual gaming job.
More than 4gb of ram will keep your lowest frame rates up.
If 6gb is good, consider a 1366 board, if you want 8gb, then a 1156 board is a better fit. Pick the CPU accordingly.
If you have both a gaming monitor and an auxiliary monitor, consider the 661 and a H55 motherboard. put your gaming monitor on the discrete vga card, and your aux monitor on the integrated adapter. Otherwise, get a cheap second vga card for the second monitor.
---good luck---