How about we start by discussing the video card requirements. Since you're looking at 790i boards I'm going to assume you're a gamer, you have a good budget, and you prefer nVidia cards. You could get one of these combinations:
1. some 790i MB + 4GB DDR3 + two GTX 280 cards - cost about 300+160+2*300
2. eVGA 750i + 4 GB DDR2-800 + two GTX 280 cards - cost about 140+50+2*300
3. GA-EP45-UD3R (or Asus P5Q Pro, etc.) + 4 GB DDR2-800 + one GTX 295 - cost about $105 + 50 + 500
All three have similar performance. In fact, all three have the same 2x240 stream processors.
#1 is about $1060. It is the best option only if you intend to add a third GTX 280. In that case add a Toughpower 1200W PSU to the mix ($370). Otherwise, it is a major waste of cash compared to the other two options.
#2 is $790. It is a waste compared to option #3, especially if you add the PSU (e.g. Corsair 1000HX, $250 at newegg).
#3 is only $665. It also has the advantage that you can get a cheaper PSU (e.g. PC Power & Cooling S75CF 750W, $80 at newegg after rebate).
#3 also has the advantage that you get a more stable P45 MB.