I'm sort of there too ... I just built my third HP XW4400 Workstation 2.3GHz Dual core. $49 + shipping for the chassis, Intel MB, 4GB RAM, DVDRW, and SATA or IDE drive interfaces. They have variable speed fans, so if SSD for HDD, no heat, no noise, no issue.
My big thing is audio and tweaking files. So sound is a crucial factor for me. Found these HP Workstations have very nice quiet sound systems (Realtek). PCIe for video board slot, so buy whatever floats your boat. Tons of ATI and nVidia boards out there. Any O/S you want. Zippy boxes. The XW 4600 is a set up
The HP Z400 (and above) are even faster, but the sound sub-system is not quite as clean or quiet ... CAD guys have been running these for years. Very stable. Six USB on the back, and two on the front. Mic, headphones jacks front and back. Don't know what else you need ...
Never met a Dell (workstation or business class) with a sound sub-system this quiet ...
I have a M-Audio board that I was going to install, but the OEM components are so close, I never bothered. Of course I don't have RCA jacks in and out with the OEM, but I can live w/o for now ...