Question to computer/hardware geeks

The fan of the PSU in my PC is now finally on its last legs. Previously I had to hand start it and it was a bit noisy but now it's just rattling slowly at times and not doing its job anymore.

1) If I simply buy a new PSU, what do I have to know about my computer so I get the right one?
Everything else is working fine.

2) Having looked at prices for cases/motherboards, I could spend a bit more, get a new case and a new motherboard. Can I just plug in my existing hard and CD drives and cards or will I have to reinstall Windows etc? The system (W98 SE) is stable enough, a bit faster would be nice, though.

3) Expanding with option 1), what can I do to my PC to make it faster apart from adding RAM? It's currently a PIII 450 on a motherboard that was new at the time, what sort of processor will it take? I have to dig out the specs but maybe someone can give me a rough idea.

Update: Thanks for all your comments.
I think I'll just replace the PSU for now as the comp is fine for what I need it for and I still have my laptop, anyway. I'll have a proper system upgrade when I have a long, free weekend in a couple of months or so. :o)

Club Hard Drive is alive!

It's on the 30th of April, at the Carling Academy, London. Anyone up for coming along?

In other news, I have found a very bad (for my wallet) website, with all kinds of interesting dark ambient and noise stuff, and at very good prices, too. Argh. Curse you, Aural Pressure!