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.
edited to remove redundant "howevers"