All you need is a new mobo, and a new chip (of course). I think you have PC133 ram, so you're okay (unless you want to get a DDR board). The power supply though, you will probably need a new power supply. What kind of slot board do you have now? What's your system like? (is it the dfi p2xbl rev.d1, the gaming system in your rig)???
You should be fine, but yeah, PSU most likely. I just got a enermax 300W yesturday for 50CND, so you can get one of those, they're good. You might want more power if you go really high, like 1.4 gig AMD or whatever. What are you planning on doing with that PIII 450? I could use it.
<edit> You'll be able to swap motherboards, and windows will probably load, but it will detect all new hardware for a little while. You'll have to reformat anyways, so you might as well do it right away, of course, you don't HAVE to.. but it'd be wise.</edit>