You would be better served with re-capping the PSU, with higher temp capacitors and adding a line filter in place of the IEC plug. It's heat and "dirty" power (a type of power ripple) that kills cap's over time. A higher RPM fan does not equate to more air. It also needs to have a higher static pressure to take advantage of the greater CFM you you are expecting. It's rather rare that the power transistors and diodes die in a PSU when compared to the cap's going out. So focus on the caps to get maximum life out of the PSU.
You can just plop a new fan in, but the price of a better fan than can really push some worthwhile air vs the original will be the same as the caps and IEC filter yet offer less value because of noise level and increased dust.