i am getting a new PSU. what is a diff between ATX, ATX12V and EPS 12v? currectly i am using ATX12v, so i have get a same kinda PSU? does it really matter? cuz i see some 600W ATX(not ATX12V) is in good price.

3.2GHZ Intel Pentium EE 840
1GB Dual-channel DDR2-533 RAM
Asus P5ND2-SLI Deluxe mobo
2 EVGA E-Geforce 7800GTX
Creative Soundblaster X-Fi
550W PSU

Simply saying it's a 600W PSU means nothing...there are good 600W PSUs & there some that are absolute garbage. Proper amperage across the various voltage rails is what's important, especially the +12v rail(s). Considering you're looking at 600W units, you're gonna have to spend a good buck to get something decent. And looking at your specs, you SHOULD be spending a good buck to protect your investment & you should get a PSU that's 'SLI Certified'. Doing a quick search at newegg only turned up one, & it's gonna cost you over $150

http://www.newegg.com/Product/Product.asp?Item=N82E16817194001