I don't know the costs your getting this for, but you may want to consider the new Athlon X2, 3800+, or 4200+.
Because your only using 2gb of ram, you could easily get away with these chips, unregistered cheap ram, and you'd still have room to upgrade to 4gb if you wanted in the future. I don't know if youd need a high end video card since your probably not watching more than 16 channels at a time?
Id Consider Building
AMD Athlon 4200+ X2 (2 x 2.2 Ghz which in test are > 2 xeon 3.4 ghz)
Asus A8N5X Mother Board
4 SATA Connectors, 2 IDE Connectors
2 GB Corsair 667 DDR Ram
2 Slots Free
DVD-R
ATI 9250 128 Mb Vcard
Many Combinations of Hard Drives, Up to 6.
You could use a WD Raptor 10000 rpm SATA, 74gb if you wanted a fast access hard drive, scores very close to scsi drives for less than half the price.
and or/ 4 x Hitachi 500GB Hard Drives
If you needed more room, you could always use the usb exterior hard drives. $30 per enclosure up to 10 usb drives.
This set up should be cheaper, and allow you to spend more money on Hard drives, or Ram, although 2 gb should be more than enough. You could even start with 1tb and add from there.