Maximum wattage on combined 12v rails
divided by 12 = amps
For example: my Dell 305w psu has 22amps
12vA 18a + 12vB 18a
264watts maximum on the combined 12v rails
264 divided 12 = 22
20a I believe is the max for a single 12v rail
You do not just add 18a + 18a = 36amps-INCORRECT biggest mistake people make
The 8800GTS 640 says 26amps (unless a heavy system load)
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=2597918&CatId=2514
Minimum System Requirements
1GB of system memory
CD-ROM drive
50MB of available hard disk drive space for basic driver installation
Microsoft Windows XP operating system
PCI Express-compliant motherboard with one vacant PCI Express x16 slot
400W PCI Express-compliant system power supply with a combined 12V current rating of 26A or more