I think newegg only exchanges parts, no refunds or credits, but you could by all means try and contact them and work something out.
Anyways, for your system, I'd reccomend bare minimums at:
12v @ 18 amps
5v @ 30 amps
However, I will say your system will do much better under a 450 watt or greater power supply, with 24 amps on the 12v rail, and 40 amps on the 5v rail.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w