This set up works like a charm! Prices quoted are in CDN $ (about the same in US$), and were valid at time of purchase.
Notes: I haven't tried the onboard Gigabit LAN yet. The mobo can be added to the compatability list.
The fantastic value Celeron 420 CPU ($37):
http://www.directcanada.com/products/?sku=12200BD0521&vpn=BX80557420&manufacture=INTEL
The CPU is essentially a Core solo, and is a great workhorse for basic tasks.
With this eVGA mobo ($60):
http://www.ncix.com/products/index.php?sku=26644&vpn=112-CK-NF75-K1&manufacture=eVGA&promoid=1019
The mobo has integrated video, giga LAN (will check if unRAID compatible). I may use the same board for my next HTPC upgrade, as it supports 45 nM quad-core CPUs, has onboard DVI (HDMI in the e7150 version of this board) and is a lot of bang for the buck.
I added the following:
This RAM ($18 after rebate in late December). OCZ Gold 6400 2x 1GB
http://www.ncix.com/products/index.php?sku=18495&vpn=OCZ2G8002GK&manufacture=OCZ%20Technology&promoid=1001
A GA-311 PCI gigabit NIC ($10, eBay)
http://www.ncix.com/products/index.php?sku=24753&vpn=GA311NA&manufacture=Netgear
Antec Trio True Power 650W ($49 after rebate in late December)
http://www.ncix.com/products/index.php?sku=24126&vpn=TP3-650&manufacture=ANTEC
Three of these WD 750 GB drives ($154 each)
Western Digital Caviar 750GB SATA2 7200RPM 16MB 8.9MS Hard Drive 3 Year MFR Warranty
Kingston Data Traveller 1 GB ($7)
http://www.ncix.com/products/index.php?sku=21439&vpn=DTI%2F1GBCR&manufacture=Kingston
ThermalTake iCage ($16). Can hold three 3.5" drives in 3 5.25" bays. Big fan keeps them cool (running at max $33 C). I could have crammed more drives in with another bay convertor, but I wanted to keep 'em cool and spaced well.
Silicon Image Generic 4-port SATA card (for use with future drives) $16 from Geeks.com. I have the drives already and will be installing them once I upgrade my unRAID basic to the next version up.
http://www.geeks.com/details.asp?invtid=CL-SL3114
Monuel Monasco Case ($59). I love it. As quiet as an Antec Sonata II.
http://www.ncix.com/products/index.php?sku=27586&vpn=Moncaso%20DVT%20201%20%28G600%29&manufacture=Moneual
Total of about $770 with taxes and shipping, or $270 without the drives! I have 4 more 750 GB drives to install shortly.