Curious, but I thought the Kingston HperX was having problems with the ASUS P800C? Last I heard look for any of the better brands (GEIL, Mushkin, Crucial, Corsair, OCZ) using matched pairs of DDR RAM. Look for fastest rating PC3500 or faster) with lowest latency (CL2 or 2.5). Just go for 4 sticks if it is single sided or 2 sticks of double sided RAM, for fastest speeds. I also hear that at or above 1000 (quad pumped 250) the latency is not so important, i.e., PC4200 CAS3 is faster than PC3200 CAS2, but that is a rumor only (haven't seen any benchmarks). Of the high speed stuff (PC3700 or higher) OCZ is the champ at the moment, GEIL is a little slower but much cheaper. All others are playing catchup, but Mushkin, Corsair and Kingston dominate at the low speed, low latency, i.e., PC 3200 area.
The MB brands are not that important, but look for a good price on a major brand (ABIT, ASUS, MSI, GIGABYTE, Shuttle, Chaintech, DFI, SOYO, EPOX, etc.) with a 875 (best) or 865PE (better) chipset that is OC friendly and has the feature set you want. Especially make sure you can adjust AGP and RAM voltages in the BIOS (download the manual and read it).
Chuck the old RAID card if you need to buy new HDs. Buy the SATA standard and get a board with built-in SATA RAID. BTW, the ASUS built-in IDE RAID sucks in both the P800C and P800P boards (I know because I have them), the SATA RAIDS are much better. Not to mention but SATA cables are much better (smaller and prettier) than IDE. Make sure you have SATA power cables too, they should come with either the MB, HD or PS or all 3.
Buy a good 400 watt or better Power Supply. Fortron or Sparkle are the best, IMO, for the money. I have one of those fancy see-through 500 watt Turbo Case Aspire PS (I found it on sale for $40 and could not resist ;-)) and it works as well as my 450 watt SmartPower Antec (also an excellent PS). Stay away from light weight junk PS's, that usually come with cheap cases.
Water cool, if you can afford it and your case has enough room. It will be much quieter and more stable the an equivalent air cooled system. I could not talk on the phone over my old air cooled system. Use nothing smaller than 3/8" inside diameter (ID) tubing. Danger Den has the best beginners kit, IMO, but ASETEK, AquaStealth, AquaXtreme, and Swiftech, also offer good kits. Stay away from the Thermaltakes and anything else with small ID tubing, they are only good for quiet stock speeds. If you don't have (or buy) quiet fans invest in a cheap fan controller, you ears will love you.
Good luck OC'n