Bidding my 939 build goodbye as my AM3 components arrived today!
Phenom II X3 720BE
Gigabyte GA-MA790X-UD4P 790X RT
4 GB G.SKILL DDR2 1066
640 GB Western Digital black
640 GB Western Digital green
2 Lite-On dvd burners (one IDE one SATA)
Multi-card reader in 2.5" bay
HT OMEGA Striker 7.1
4 120 mm fans
550W Corsair PSU
Now my question is: Would my PSU support a 4850 and 4830 in Crossfire, or would it be worth it to get a 4870 or 4890?
You can crossfire two different ATI cards (not NVIDIA though). 550W is the minimum recommended PSU for dual 4850's in crossfire. If you crossfire a 4850 with your 4830 you should remain within the power envelope. With that Phenom running 95W, two HDDs and 2 DVDs + 4 case fans, you might be stretching it close, but I wouldn't worry.
I am running an overclocked Athlon (probably around 95W), single 4850, 4HDDs, 2DVD, and 4 case fans on a 450W PSU, so the minimum often works out fine.