I'm planning to buy a new power supply and am leaning toward one that is modular. Would I be better off buying a PSU with two rails or one?
My current system is a Dell Inspiron 531 with
AMD 2/x64 5600+
Geforce Zotac GT 240
PSU: 300 watts
2 Optical Drives
5.1 sound system
3 additional fans using external power source
24" HD Viewsonic Monitor
My goal is to get an HD graphics card (Geforce preferred) that supports 3D graphics and a PSU of at least 650 watts.
It doesn't matter as long as the total wattage on the 12v supports the graphics card or cards that you want to use. The 12v power is the MOST important thing to consider (other than reliability) when buying a PSU, even more important than the PSU's total wattage.
When a card lists "minumum x watts" it doesn't mean the PSU's total wattage. It's referring to the wattage of the +12v rail specifically.
If you get a PSU with 2 12v rails and neither one has enough wattage on it's own, you're boned unless the card has 2 6 pin connectors.
Ditto if you have a single 12v rail that doesn't have enough total wattage for your (hypothetical) SLI'd/xfire'd set of cards.