I forgot about it and then i couldn't edit the thread. It's a GeForce 9800GT. When i upgrade the GPU to 6870 or the 6950 i will upgrade the PSU, too.
If what you have is the Enermax Galaxy EGX850EWL, with its combined +12 Volt continuous current rating of 68 Amps and with three 6-pin and two 6+2 pin PCI Express supplementary power connectors, there is really no need to replace it when you upgrade your graphics card to a Radeon HD 6870 or a Radeon HD 6950.
It already has the 24-pin ATX main power connector and 8-pin ATX 12V power connector required by the GIGABYTE GA-P67A-UD4-B3 motherboard.