Power supply/video card question

Right now I am really trying to play left 4 dead. the first thing i did was buy a geforce 7900 gt, well it turns out my power supply isnt big enough to run that type of card. Im using an acer m5100 that has a puny 250w power supply. I found a decently priced 500 watt one but I just need to know what I should be looking for in the realm of rails. some say it has 2 12v rails and some say single. What do I need or how do I find out. Any help would be greatly appreciated.

Sorry its a amd athlon 64 x2 dual core processor 4400+ 2.29 GHZ 2.75 gigs of ram 300 gig hard drive.

Proud owner of a Antec EA-500D that provides 500W, with 2 +12V rails each at 22A, running at 83-87% efficiency. That should be more than enough for the 7 series, even the 8 and the 9 and the GTX 260s. It comes with lots and lots of connectors, (2 PCI-E, 4 SATA, 6 Molex, 1 8pin) so its a pretty good bet. Made by Delta (which is pretty big in industrial level power supplies).

You should be able to find that in the states for approximately 50-60 dollars. If not, I think newegg has it for about 90.

Theres also Corsair, PC Power&Cooling, OCZ, all of which will perform well, given you get one with enough power.

In terms of 1 or 2 rails, it really is irrelevant unless you are planning to SLI with serious high end cards.
This forum thread (article) should give you a good idea about the whole 1vs2 12Volt rail controversy.

http://www.jonnyguru.com/forums/showthread.php?t=3990

Enjoy