NVIDIA have traditionally had better drivers than AMD for their professional (Quadro) graphics cards.
Soft-mods are a dicey affair, IMO. I would strongly suggest the Quadro 2000 or 4000 instead, unless you need 3 monitor Eyefinity support with a single card.
What you could do is try out the Quadro 2000, and if your client finds it to be inadequate, you could return it for a minimal restocking fee. Then you can switch to a soft-modded card or a higher specc'ed one. Or you could do it the other way around.
Don't take FSP power supplies. They aren't very good. Stick with Seasonic or Corsair, or maybe an Antec. I use
www.jonnyguru.com for PSU reviews. Maybe you should allow some overhead for expansion - SLI / Crossfire in the future. You can use
this to calculate your PSU requirement.
Just how do you plan to implement this HDD caching plan?
AFAIK, RAID 0+1 and RAID 1+0 both require a minimum of 4 disks.
The WD Caviar Green is one pathetic HDD. Strongly suggest a 7200RPM WD Black or a Samsung Spinpoint F3. Shouldn't need more than 1TB. IMO, the HDD would probably not be able to keep up with the SSD's in RAID and probably bottleneck them to a significant extent. I'd suggest you use the 2 SSD's as the boot drives in RAID1 with a two regular HDD's meant for data in RAID1. You might consider waiting for the Intel Z68 chipset which willl enable SSD caching.
I don't think there is a need for a water cooling circuit. There are some pretty good coolers out there (like the Venomous X and Prolimatech Megahalems) which would allow you to do 4GHz+ on air easily. On a budget, the Scythe Mugen 2, Zalman CNPS 10X Performa, CM Hyper 212+ and the Xigmatek Dark Knight offer value that's hard to beat. AFAIK, you shouldn't have a problem OC'eing with any of them up to 4GHz.
I second the choice of motherboard.
If you're going to be overclocking, you should get RAM clocked at-least @ 1600MHz or higher. The principle is that the faster your CPU works, the more data it needs, so a higher bandwidth RAM is necessary to prevent bottlenecks. Here's a
link.
Hope this helped.
Adios.