2) If your acbel is a acbel Polcom then it is a tier 3 PSU which means it is exceptable quality - Myself I prefer my PSU be rated as tier 2.
IE a Tier 2, 450 Watt PSU would be better than a 500 Watt tier 3.
Tier List: http://www.eggxpert.com/forums/thread/323050.aspx
Looks like a tier 2, 450 Watt PSU would even support two 7790's in xfire
@ delmetor "It needs 21A and 500W min." I'm guessing you got that from GPU Manuf. Their ratings are based on about a tier 4 PSU which may only deliever about 60% of rated power under load!!!
NOTE: Minor fib by manuf: two +12V rails @ 18 Amps. This is NOT really true. IE if you did draw say 17 Amps on V1 then V2 would be limited to < approx 8 Amps as Total power can not exceed 350W (ie (18 + 18 Amps) * 12 V = 432 Watts, YEA, give me a break, from a PSU rated @ 350 Watts.