Watercoolers are something I think should be built by the user. THe kits are overpriced and don't deliver the performance a home built kit does. Right now I have the heatercore of a Chevy Chevette, 4x 120MM fans, 2 sucking 2 blowing on the radiator. Have a 2 liter resevior, all of this is in its own box, and is run through 3/8'' tubing to the case. Have a dangerdin waterblock for cpu, and run that parallel to one on the northbridge then to the graphics chip and memory. The name of the pump escapes me, but it is 300GPH. Very quiet. I use deionized water, so even if it does spill out, there is a very slim chance it will electricute anything. Don't have any coolant or die in there right now, but might later.
As for a computer built if you want high performance, go for 2 SATA drives, and put them in RAID 0. You can get this off of any canterwood, springdale, or KT600 chipset. I remember nvidia spouting off about SATA, but don' know of any motherboards that use it.
For processors, I would recommend
for intel: 2.4C-2.8C, anything over that is overkill. If you intend to wait for the prescotts, you will be waiting until mid next year until they are in socket T and under 500 dollars a pop.
for AMD: If you are an overclocker, there are 3 chips I recommend. 1) the athlon 1700+: this chip is very cheap, can reach speeds of 2.2ghz on air (with a slk-900U), which is something like an 86% gain. Overall a good overclocker 2) Athlon 2400+, this chip is a more consistent overclocker, and I have seen it's mean overclock at around 2.3ghz 3)Barton 2500+, this chip has the added bonus of the extra cache, so even if it does not reach the highest speed, it can still be as fast. I have seen the bartons get to 2ghz-to 2.5ghz, 2.2ghz can be done on air, again with a slk-900U. The highest I have seen with the 2500+ is 2.9ghz, with a vapor chiller.
Motherboards: For intel: Canterwood and springdale, really are basically the same thing. It comes down to which features do you want. Both now have PAT (basically anyway), and both are decent overclockers, minus intels chipset, which is made for stability. Boards I recommend are Abit IS-7, IC-7. ASUS P4P800, P4C800, Albatron 865PE PRO.
AMD: I really like the new Nforce 2 400 chipset. It is single channel, but is faster than the dual channel set up. Nforce 2 Ultra 400 is good, and the asus A78X Deluxe from ASUS is a top favorite. It comes with 5.1 sound, dual LAN, and is fairly easy to overclock with
RAM: A lot of people will tell you how great Corsair is, but frankly I despise it. I would go for Kingston HyperX PC3500, Micron PC3500, or OCZ PC3700 Gold. I have also heard good things about Buffalo PC3700 if you do the voltmod on the P4C800, capable of getting absurd timings of 2-2-2-6.
Sound: If you are big into sound and want to wire stuff into your computer, go with the soundblaster audigy 2 with the front panel. If you want quality sound, go for the m-audio revolution 7.1 If you want gaming surround the soundstorm on the nforce 2 really is great.
GPU: Graphics cards have been a big topic lately, especially with all the nvidia optimizations getting caught. I personally would go for a 9700Pro, or 9800Pro for high end graphics. If you want midrange, go for the 9500Pro, because it is actually more powerful than the 9600Pro. Also have seen some good things about 5600 Ultra, but wouldn't take it personally.
That will be all for now. If you have questions, just ask.