The Opterons are made from a slightly higher quality silicon compared to the San
Diegos (so I hear). You'll do fine with either. The 3700 Sandy will allow you to use the 11x multiplier unlike the Opty 146.
The DFI NF4 Ultra D is killer but it can be upgraded with an "Evercool chipset cooler" for about $8.00 (easier to install before mobo goes into case).
With the mobo, go with quality ram (I recommend GSkill PC4400) and a Big Typhoon cooler like my rig profile. The Power supply must be 480watts or higher
and I recommend OCZ 520 Powerstream.
The rig can be built at fairly low cost. gL