You can use plain ol drives, it won't matter in the end because the bottleneck will be the network connection, which is
vastly slower than the drive throughput.
The RAID controller I use is the Fasttrak SX4. Works great. I have a question though....32GB each? Really? They have low reviews, looks like crap IMO. You're better off with 4x750GB drives in RAID 0+1. Even with gigabit, your network will still be the bottleneck.
CAT6 will do, but so will CAT5e. Don't put any tight bends in the cable or it will drop your speed. Sloping curves, no bends. Nothing parallel to power circuits, either. RAM and CPU speed will not matter much, unless you handle very large files. In that case, go with the fastest RAM. You could underclock the CPU quite a bit and still feel no effects.
If you do lots of random writes and reads, the flash drives will make you whine.
If you still want super speed, these will do the job for random writes over the flash drives...
Newegg.com - Western Digital VelociRaptor WD3000GLFS 300GB 10000 RPM 16MB Cache SATA 3.0Gb/s Hard Drive - Internal Hard Drives
Newegg.com - Western Digital VelociRaptor WD1500HLFS 150GB 10000 RPM 16MB Cache SATA 3.0Gb/s Hard Drive (bare drive) - Internal Hard Drives