This is my computer build - HP Pavilion Elite m9180a
The only thing that is different from that link is the GPU has already been upgraded to a NVIDIA GT240. When I got that upgraded though someone mentioned I might need a better power supply or fan but wasn't sure.
apart from psu, the gtx650 suggests you might be gaming, if so the e8400 might be the bottleneck in some games, maybe not if you are gaming on a smaller 15/17" lcd/lower details, and memory is probably slower DDR2, but once you change cpu you need a new mobo, new cpu, new ram, new psu(maybe), gets pricey, in some cases it's not worth it, you should just sell off parts and buy a good new/used pc
3GB RAM + an old Core 2 (not even a quad) will heavily gut your FPS and especially minimum FPS in any modern AAA game from the past 2yrs, and a GTX 650 isn't a gaming card. That 240 is sufficient for 1080p porn and office work so there is no point in upgrading. If you want to game, sell this off and build a modern 2014 box.
I once had an overclocked (3.8ghz) E8500 with HD5850 and found that the CPU was actually heavily bottlenecking the card, particularly on online shooters like Battlefield 3 and it was also struggling on to run Starcraft 2 on the highest settings. It was only after an uprade to a Sandy Bridge i5 2500k I was actually able to fully utilise the 5850.
I would not upgrade the machine, but probably put the cash towards a completely new build. You don't want to throw any more money into a LGA775 machine that can't take DDR3 memory, PCIE 3.0 or SATA3.
would of said Power supply - PSU
300w is the minimum suggested specs for your GT240
300W would be on the light side for the GTX 650 it falls below the suggested minimum specs of 400w
http://www.geforce.com/hardware/desktop-gpus/geforce-gtx-650…
this said should still work
slightly larger PSU wouldnt cost alot though replacing it may be challenging without having to strip the whole system out.