Do I need PSU upgrade?

[AceIlluminati]

Current System:

Motherboard: ASRock X38TurboTwins
Processor: Core 2 Duo E8400 3.0GHz
GPU: Sapphire Radeon HD 4850
RAM: 8GB (G.SKILL 4GB (2 x 2GB) DDR3 1333)x2
1 140mm fan, 2 120mm fans, one has LED
DVD-RW
2 SATA 7200 HD, 1 IDE 5400

PSU: Silverstone ST70F 700W - http://www.newegg.com/Product/Product.aspx?Item=N82E16817256045


All I want to do is add another HD 4850 in crossfire. I think I'm ok on watts..but I still don't quite understand the amps for the 12v rails, there are 4 of them at 18.0 each. Do I need a bigger PSU? Thx..

 

[AceIlluminati]

Thanks.

 

[AceIlluminati]

Just curious, do the rails work together in my PSU?

Like if its 18.0 per rail, does whatever plugged into that rail only 18 amps are available too it? Or does it borrow from other rails if needed?