Hi,
I've been posted a thread related to this, about new build PC system for gaming. I have budget about $1200 (for now), and i will use  
 
processor   :  AMD Phenom 9600     US$  249 (maybe OCed)
Mobo          :   ??????                       US$  150 (max)  
Memory      :  Corsair TWIN2X2048-6400C4DHX     US$ 72
Gr. Card     :  HIS HD3870 IceQ 3 Turbo 512 MB DDR4    US$ 230
Harddisk     : WESTERN DIGITAL WD3200AAKS US$ 86  
DVD            :  ASUS DRW-1814BLT          US$ 43
PSU            :  CORSAIR CMPSU-550VX          US$ 104  
Case          :  COOLER MASTER RC-690      US$ 99
All total is $883
Thats my last system hardware, but now i wanna try a Crossfire configuration For future investment (since i've (finally) got Q6600 with 8800GTS (G92) SLI configuration). Any recomendation, critics are welcome.

Wrong title. Should read "Phenom on Rehab..."
 
Best AM2+ o/cing mobo would be MSI K9A2 Platinum for about $150 US. Shop around.
 
You can do single, dual, tri & quad CrossFire on 790FX. That's the beauty of it. Simliar to the new 790i with single, dual or tri SLI. But you'll have to make sure the gpu supports the config.
 
Everything except the psu is fine. I don't know if you can do quad CF with 550W.

Oh... sorry about the title akhilles. You recomended MSI K9A2 Platinum, i've checked that mobo on the site. Pretty cool i think, but in Indonesia that mobo priced for $ 205, $55 more!!! For that much money i could get 120G hdd and for the psu i'm gonna considering change it to 800W. Anyway, for stability for OCing, i can sacrife $55 (even more). Thanks for the input, maybe anyone could correct any of my hardware specification even critics are open.