what is your price range?

have a look in your motherboard manual, it will be in there somewhere. If it didn't come with one go to the manufacturers website and download it.

id like to see it become pratical in australia with our tiny download limits which still cost a large amount of money, its cheaper to buy GTA 4 each month than to pay for the internet and onlive

Could there be something wrong with the PSU? or could something in the motherboard have broken when you installed the fan or could something be short circuiting it?

switch out the GTS 250 for a 4850/4870 (they are alot faster and getting alot cheaper and switch the Q8200 for a intel E8200 or Intel E8400 then overclock it. The dual cores are still faster with most things than dual cores and they can OC quite easily with adequate cooling. I have a E8400 at 4ghz right now and it stays below 50c in brisbane.

The windows 7 rating seems to be better that vistas one. When i overclock my cpu it goes up by a fair bit

If you haven't already tried overclocking the GPU, give it a go. I don't own a GTX 260 but ive heard they overclock quite well. You may get a bit more time out of it and you can save up for the next generation of GPU's.

how about going with a 4890 or something similar and putting the leftover money to get a better monitor

Is the fan connected to the motherboard, and did the heatsink come with thermal paste, it did for mine nd if not did you apply it properly. these would cause massive overheating issues with your CPU and would automatically shut down your computer

Something probably got fried in the motherboard. If you have one try another sound card and see if that fixes the problem