Ehrm, you
might get away with plugging a 8800GT into that, but I would really recommend a 500W or above.
I have a 520W, running a 8800GTS512 (one notch above GT), Quad-core @ 2.8GHz, 4GB RAM, 4 HDs, 1 optical drive and a X-fi soundcard. It's sufficient, though it begins to struggle as soon as I overclock the CPU (actually starts putting out hot air at that point); I would say a 500W is your minimum for a decent gaming rig.
A
good 600-650W supply should run you about 80-90 quid, depending on brand (avoid the cheap ones).
Putting it in is a matter of loosening 4 screws at the back, disconnecting all the cables, put in the new one and connect everything back up.
Something to keep in mind is that modern boards have a 24-pin connector on the board, where older ones have only 20. A lot of modern supplies have a 20+4 pin connector, meaning they can connect to either a 20 or 24-pin board.
http://i1.tinypic.com/ncxcf5.jpg
Just check what connector is on the board and make sure the supply has a suitable connector.