and what power supply would it need?

Well the 8600 doesn't use that much more than the 7200, about 25-30w. Best use the extreme calc link we provided earlier for a recommended power supply calculation. I may have mentioned this before, or not, but power supplies degrade over time (capacitor aging), and consume more power. What may just about work now, may not a few months down the line.

25w-30w? my power supply is a 300w. somethings not right here.
which is better card the 7900gt or 8600gt? what power supply would be necessary for the 7900gt?

No I meant 25w-30w more than your current graphics card, and I think your also forgetting the other hardware on your PC. Like I said before, check out eXtreme Power Supply Calculator Lite v2.5 (this will give you a recommended estimate of how much power you'll be using).

okay well i think its just easier to get a new power supply. a 400watt one is only about 15-20 euro on ebay.would that be a reasonable choice? would it put me in a safe zone regarding the 7900gt?

Definitely. One thing to note about the PSU (as well as many other PC parts) is that it's important not to buy any cheap old power supply that happens to fit in your PC. There are some nasties out there. The PSU is the basis of your PC, and I'm sure if ih8bills stumbles across this thread, I'm sure you'll hear a long long speech about the importance of a reliable PSU to put you to sleep lol.

An example of a reliable brand of PSU would include Thermaltake, but I can personally vouch for anything you buy from this web site: Quiet PC UK - Quiet PC Power Supplies for a Quiet PSU

^ an absolutely amazing site for PC parts, very good quality. You may spend a little more than you planned, but it really is worth it.