The PC Power & Cooling will definately be good for the Dual Xeon rig. Plenty power. It's a highly reliable PSU and rock solid. It's hard to find a better one.
For the other rig, I'd say the OCZ is indeed a good pick. The 600W version probably is enough already, but for $115 after MIR, the 700W GameXStream is a good pick. It's a good PSU, just lacking the proper connections for the Dual Xeon rig. For the Core 2 Duo rig, it's plenty powerful enough. I have the FSP Epsilon 700W for my rig, and the OCZ is identical (built by FSP). See what it powers for me
(E6600, dual X1900's, 4 WD Hard drives)
Regarding RAM, depending how much you're willing to spend. I'd recommend these kits, depending on budget:
- low end: G.SKILL 2GB (2 x 1GB) DDR2 800 (PC2 6400)
- mid range: BUFFALO Firestix 2GB (2 x 1GB) DDR2 800 (PC2 6400)
- high end: Crucial Ballistix 2GB (2 x 1GB) DDR2 1000 (PC2 8000)
$307.99 - $25 MIR
given that you don't plan to overclock too much, I'd say either should do fine. The Buffalo kit is nice, since it does have the nice Micron D9's and it's not too expensive. In either case, I do recommend at least DDR2 800 (especially since the cheapest kit I recommend is not much more expensive than DDR2 533 or DDR2 667)
Regarding your motherboard choice: If you plan to run those hard drives in RAID... Do NOT get the Gigabyte DS3. The DS3 only supports RAID on a secondary controller over the PCI bus, limited to 2 drives in RAID0, RAID1, or JBOD. Somehow I am guessing you plan RAID5. The P5B-E and the P5B Deluxe would work just fine for you.