I personally have the P3V4X with a P3 550mhz (not overclocked),and a CL annihilator2 (overclocked to 230/378) and i like it alot.
I run it in my fully modified Q600n In-win tower with a powerman 300 watt P/S. I also running 6 panaflo fans ,a dxr3 decoder,SB live value,a cd burner,a zip drive,and a 7200 rpm ata/66 maxtor HD and have done lots of multi-tasking without power troubles of any kind.
I highly recommend buying the full tower over a mid-tower for the fact of cooling, and having plenty of future upgradablility. You may think you dont need the extra size.....but after you buy a small case......something will come up that you wish you had the room for. (you know ...murphys law?)
Anyway,I say its like a HD.....buy the biggest you can afford! BUT, even i have to stop at the size of the ADDTRONICS monsters....thats too big even for me! =)
My inwin is about 24" tall which is alittle tight under a normal desk.
Another great case thats alittle shorter is the ANTEC SX-830/1030 series.....these cases kick butt too.....but lack a slide-out MB tray (like my in-win), and you cant remove every piece to it. the right side is riveted and so is the top.
The in-win Q500n/600n can be fully dis-assembled which is very handy to modding it.
Like i said i got 6 panaflos in mine ( 2-120mm,2-92mm, and 2-80mm)
I even replaced the power suplly fan with a 80mm panaflo.....my case is very quite considering the tornado inside!..=)
Final thought.........SUPERSIZE IT~!