So, I have a 500W power suply with two HD 7950 200w. I'll buy more 4 GPU, but I don't want buy a 1000w power supply because the price. Two 500W power supply high quality I'll save money 40%. It's better?
You need to look at the PSU's actual specification on it's 12 volt rail/s. Different manufacturers spec their PSU's differently and mid range PSU's tend to advertise total watts across all rails (12V, 5V, 3.3V). So it may be that your 500W PSU only gives <400W on the 12V rails.....
The >1000W PSU's know they're targeting for GPU's and will tend to advertise 12V rail watts (or at least that's what I see on my Silverstones)
It's well worth purchasing a 1200W+ PSU if you can. There's also might be an electrical danger in bonding multiple PSU's, paticularly of different brands that I can tell.
@Rayhan you can purchase a server PSU at eBay and use it to power your extra GPUs. There is a lot of HP, Intel, etc. PSU for that purpose available. Look here to understand how it works: http://www.gigampz.com There also a few discussions on bitcointalk available. If your budget is tough, there is always options like $10-15 per 835W server PSU available
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The >1000W PSU's know they're targeting for GPU's and will tend to advertise 12V rail watts (or at least that's what I see on my Silverstones)
It's well worth purchasing a 1200W+ PSU if you can. There's also might be an electrical danger in bonding multiple PSU's, paticularly of different brands that I can tell.
This is PSU:
http://m.ebay.com/itm/CORSAIR-Builder-Series-CX500-500W-ATX12V-v2-3-80-PLUS-BRONZE-PC-Power-Supply-/331784243814?nav=SEARCH