Adding a cpu halved my rendering time.
FYI for those interested...
I used Ian Pellant's ray-tracing test file. I ran it after a restart but didn't close any background programs. My rendering time was 2:31. I added another CPU and with no other changes, the rendering time went to 1:16 (way faster than I expected!).
Dell Precision T5400
CPU: Xeon E5430 (2.66 GHz quad core) (now x2)
Memory: 8 GB
Graphics Card: NVIDIA Quadro FX 3700
Hard Drive: WD Black 7200 RPM 1TB
OS: Win 7 Pro 64 bit SP1
Chief: X4
I have ordered another matched pair of CPUs in the same family (Matched Pair Intel Xeon 3.16/12/1333 SLANP Quad-Core Processor X5460), but are faster. I will let you know if that change affects rendering time. I'm also going to switch to a solid state drive (Mushkin Enhanced Chronos MKNSSDCR240GB 2.5" 240GB SATA III MLC Internal SSD) and will find out if that lowers the rendering time as well.