Well since some MORON told me I don't know anything earlier in this thread I am probably not qualified to answer your questions but I will give it a try.
1. You can use your current case with the p4, Asus as well as MSI has made a backing plate to help with the cooler support (although I can't see where you would need this extra support the coolers that cool the p4 are lighter than the ones I used on my classic athlon Intel is again looking towards the future.)
2. You can use a regular PSU to run it as well. I am running mine with a 303 antec as I type this (quite ROCK even overclocked I might add

)
3. Since I don't do rc5 cracking, M -flops, intense mathmatical calculations etc. I think the p4 is incredibly fast (for the stuff that I like to do) and one of the most STABLE SYSTEMS I HAVE EVER SEEN!!
It took only about 2 hours to put my system together yesterday and it runs perfectly full of cards no IRQ battles or anything. Not one BSOD during the entire setup process. The p4 is as easy as any other Intel system is, to configure and make stable. Not to mention this is a FIRST REVISION product that is every bit as STABLE as the other Intel setups I have had that were much more mature
I know that you guys will tell me that it doesn't make any sense to upgrade to the p4 right now but it really doesn't make any sense to upgrade to AMD either since I am sure that they will change their platform soon as well (about 4 times or so in the last year I think??) and for what I paid the p4 was a good move I can stay with what I have currently for a long long time.