I am not sure how noisy they are, but I have seen new computer power supplies with high quality ball bearing fans that are supposed to be much quieter. A good high quality power supply usually runs about thirty to fifty bucks and is relatively easy to install. No need to replace the entire case. You may also think about buying some of the dynamat? sound absorbtion material and line the computer case.
As for the cooling fan on the actual cpu, they tend not to be made to very tight tolerances so it might be beneficial to replace that one with a newer one to see if it is more quiet. That would be a under ten dollar experiment.
The power compact lighting hood on my fish tank uses a standard computer fan, which I took apart and clipped the blades (a lot of the noise was from the blades scraping the outer edges) and lubricated with a teflon based oil. Worked okay for a couple of months.
I just did a computer power supply switch out, so let me know if you have any questions on how to do it.
Good Luck and Regards,
nathan