2.4GHz Dual core, no OC
prob 64 or 128bit GPU
XP pro 32bit
my PSU is prob 500Watt
Try to find out exactly your PSU so we could evaluate if you need to change it or not. (Try to check 12V rail combinated amperage)
Check the GPU exact version so we check if you're doing a parallel upgrade or not.
Check the CPU exact version.
Get rid of XP 32bit, go for windows 7 64bits.
What will be your main usage? Which games you going to play?