If I'm reading your post correctly you have a single 12v rail with 20A? Hmm the 8800GT isn't very power hungry but that is a little low. Your brand of power supply will make all the difference in the world as to whether or not it can handle that load.
That's enough power for a single card. Well that's assuming that your PSU is somewhat decent and can sustain something close to its rated power. Really though you may as well go with a 9800GT since it's basically the same thing but generally a little cheaper and easier to find.
Thanks for the replies guys!!! So i should be fine then? I said 8800gt because i can get one for £75 at a local store, My current hd2600pro 256mb is starting to show its age. Would it be fine to run with my current psu till I can get a new one in a few weeks?
Tough call... It'd be close, especially with any overclocking on the CPU or 8800GT, and dependent upon additional devices. An 8800GT draws almost 2.5x the power of a 2600Pro at full load. That's quite a difference.
It might be better (safer) to go with a 9600GT instead. You'd lose a little performance compared to a 8800/9800GT (they are the same card), but it draws only 2x the power of a 2600Pro. If you think your system can handle the power requirement of an 8800GT, wait for the ATI 4770 to come out in a couple weeks. It's priced at $100 and ATI claims it beats the 8800/9800GT. Current wattage ratings for it show as 80W, identical to the 9800GT. Could also consider a 55nm version of the 9800GTX+, also rated at 80W...
I just don't know if your system is actually capable of sustaining that level of power. If you're getting this 8800GT super cheap, I say try it, but don't be surprised if you see system crashes in games while it's under load.
Think about it: If an 8800 gt really needed 26 amps like it says on the box, that means it would require 312 watts (12v * 26a = 312w). The motherboard PCIE slot supplies 75 watts of power, and each PCIE connector from the PSU is officially rated 75w (though unofficially it can do much more). At 26 amps, an 8800 GT would need four 6pin PCIE connectors from the PSU to work within specifications. It gets even sillier when you look at SLI. At 26 amps, the cards would eat 624 watts and require a whopping total of eight 6pin PCIE connectors. That's obviously not how it plays out in practice.
OP, your PSU is rated high enough, but whether or not your PSU can actually do what it's rated is another question. If it's a cheap 400w, then it might be dodgy, but if it's brand name then you will be fine.