Need couple suggestions

I would personally recommend you the X1900XT. May be as expensive, but i feel it will last longer than the 7900GTX, especially in terms of shader-heavy performance. Besides, the X1900XTX is even more powerful and can perform as well (if not better) as a SLI config with two 7800GTs. I know nVidia cards are powerful, but at the moment, ATI are slightly better with the X1900 series, which i would see still holding out strong come around a year's time. For the MoBo, go Asus, though if you are going to go with a single ATI card, go with an A8N-E, but if you are going Crossfire (eventually), go DFI... more stable than Asus. Finally, coming to memory, stick to Corsair. They are fast and reliable, plus awesome to OC.

I never really consider Opteron but I'll definitely take a look at it. Is the performance as same or even better? Im a casual gamer but still I kinda want the best all around.
Looks like ill take the DFI mobo since it performs almost the same as Asus and its a little cheaper. Plus I won’t be going SLI soon, so i can always wait for a better mobo to come out
About the video card, I already bought the 7900 GTX and it performs a little better then the x1900xt.
About the memory I always used CORSAIR but I read some good review about OCZ. So im still kinda unsure about it.

I have the K8N Diamond Plus with a 4400+, wonderfull board, SB Audigy built in is great. Lots of bells and whistles. Any of those three motherboards are good to go with though.