Replacing the PSU is a relatively easy process. The only real trick is to remember where each connection attaches. Taking pictures (before and during removal) can be very helpful for the installment process.
Hello! I have an HP P6-2056sc and i want to upgrade the psu to a Corsair CX430v2 so i can install my GTS 450. Is it easy to replace the psu on this computer?
Try installing the card with the original PSU. A GTS 450 only requires one 6-pin added power plug. You can get a molex-to-gpu 6 pin adapter if you need to, but I think that the 300 watt PSU will likely power this card. If it has the 6-pin connector than this can't get any easier...
As C12Friedman says, if you have to swap out the PSU, you will want to take pictures first. Lots of parts and brackets will need to be removed to get the power leads out of where they have been tucked into. It's not hard, but will take some time.