If your building it this summer the possabilty could be either way as things are changing on both sides right now.
AMD- 64 bit option.
Expandability for future 64-bit applications, but by that time the processors will have changed format, infact a new pin set is coming this late summer I believe. These are less expensive to build, procs are comparible at times but hardware is cheaper. Some will say that these are better for big media projects also. AMD's throw off more heat than Pentiums though. These are supposed to be single task monsters from what I read~ What is important to understand is that although it is 64-bit. Programs only use 32 right now, and relatively near future. The 64-bit allows for slightly higher memory bandwidth too.
Pentium 4 Pentium 'C' chips
The new 'E' chips are just not conventional in my eyes....yet. They are Hot, Energy suckers and have not shown ANY benifit to 'C' chips beyond crunching SETI or Folding (due to 1 meg cache, which is actualy available on the Extreme Edition 'C' chips). These are stable chips. They have been around, and have been killed so we know the limits. Pentiums Cons are Price, and socket age. They are more expensive on the whole, and this socket set (478) is proly on it's way out. Here are the goodies though. They O/C like mofos (2.6c's will see 4Ghz with right set-up), you get Hyper-Threading (Seamless multi-program tasking) dual channel memory (efectively double memory bandwidth), and lower heat than competators. On the last point, I use a stock HSF to o/c a 2.8-->3.3 and get good temps. A good air system gets you a long way with a Pentium set-up. Also one should note that Pentium is releasing a new form factor this summer. BTX. If you haven't looked into this, do it. SOMEONE finaly pulled their friggin brains together and designed a form factor that uses physics to its advantage with the RAM and CPU down low and in front where the fresh, cold air is.
For both we have DDR II coming~ more bandwidth with stable timings, and lord knows what else will be here in the next 2 months. AMD might shock us with something new, Pentium might do the same with a chip thats NOT a failure..lol.
I was an AMD guy, they are stable as hell, and work horses. But I wanted flexability and the ease of quick o/c'n, and at the time Pent was the only one with 800fsb that caught me.
Ultimately keep eyes and ears open for your own opinion. If your a multi-tasking O/C'er, go P4C, not even AMD'ers can argue with that, not if they are honest with themselves. But that is the current situation, I can't stress enough of what this year hold in changes for our community. No one knows. I hope this helped.