Should I bother to replace the power supply?

So yeah, a few weeks back my computer started making this irritating, loud sound, like one of the fans was fucking up.  I replaced the CPU fan, but it's still making noise, and since I don't have a graphics fan it's probably the power supply fan.  If it was just the noise, I would ignore it since it does eventually go away, but recently my computer has been slowing down rather badly.  Can a power supply cause this, or is something else getting screwed?

if the power supply fan isn't fanning properly, the power supply is probably overheating.  overheating can cause any number of problems if it gets so bad as to affect the chips on the motherboard.

you can probably just replace the PSU fan without buying a new power supply.  hopefully there was no permanent damage to the motherboard, otherwise you'll need to replace that as well.