If you are considering a Athlon Tbird (which for some strange reason has outperformed the P4 with similar setups) I would look into that, because AMD currently are much cheaper, but performs as good or better than the Intel product.
I would stay away from DDR memory at this point, as the performance gain, and the price are not exactly worth it at this moment, for the best memory in my opinion, I would pick up a stick or two , of 256Meg PC133 7.5ns CAS2 memory (you can get ECC as well, slows you down just a tiny bit, but ECC = Error Checking and Correcting, good for a more stable system) from crucial.com.
my system specs are (and I am a developer, gammer, and Audio listening as well)
AMD Thunderbird 1Ghtz (200FSB)
(2) 256Meg PC133 7.5ns Cas2 ECC memory
126Gigs Total HDD space (60G ATA/100, 61G ATA/66, 6.4Gig Backup drive)
5xDVD/32xCD
16x10x40 CDrw
Visiontek Geforce3
SoundBlaster Live! X-Gammer 5.1
[with Cambridge DTT2200 5.1 Speakers]
Maxcool server tower case
10/100 nic for cable.
etc
you can see pictures of the system on my site (my signature) should be under "gallery" as "current machine..."
Prior to the K7 (athlon classic) I've been pretty much with intel, since the K6-2 were in my opinion as stable as a tetonic plate. The Duron is also a very good option to consider, the duron is the same as the Tbird, only with 64K cache, instead of 256K, much cheaper in price.
The watercooled system I had was a Duron 800. I got rid of that guy, as I felt more comfortible with air cooling, also wanted a faster processor

Karl Blessing aka kb244{fastHACK}