Power supply questions

Hey everyone

Just have a couple of questions relating to an upgrade I'm thinking of doing.

First, some background info:

A couple of years ago I built a new computer with the following specs:
ASUS M2N32SLI-Deluxe motherboard
AM2 Athlon x2 4600+ processor
2gig corsair ram
7900gt graphics card
Super Flower 600W power supply

It was my intention to get a second 7900gt when they further reduced in price, but that never ended up happening.

I have since upgraded to an AMD Athlon x2 6000+ processor, and my graphics card died, so I am using friend's old 6800gt.

I wasn't playing too many games with uni and stuff, but now I'm on holidays and am looking at upgrading. I'm thinking that since I have a SLI capable computer, I might as well go for 2 cards and am looking at 2 x 9600gt.

However, I don't really want to have to upgrade my PSU as well. The 600W power supply has 2 12v rails with 22A max current each.

From the research I've done (as well as putting the components into the be quiet! calculator) I'm fairly sure the PSU can handle it, I was just hoping a few people could confirm this for me.

Also, could you clarify something for me about the workings of a power supply?
I've read that the 2nd 12v rail is used to power the cpu through the motherboard and the 1st is used to power everything else, according to the ATX standard. Does this mean that everything on the motherboard is powered by the 2nd rail? I know that the PCIe slots can only supply 75W and the 6pin connector supplies the remaining required power (up to 75W) for the graphics cards. So does the 2nd rail power the cpu and part of the GPUs, while the first rail powers the rest of the GPUs as well as hard drives etc? That would make more sense to me because the CPU is only 120W (I think).

Any comments/opinions welcome.

Thanks

I would go with a GTX260 216 instead.