I Googled for Precision 390 and found Dell shipped these machines with Q6600, Q6700, QX6600 and QX6600 processors.
My guess you could use any core 2 duo or core 2 quad 65nm CPU with a 1066 FB.
Check out the sites below
There are 2 revisions of the board A00 and A01. The revs had nothing to do with CPU support though. The 975x chipset although not clearly documented supports some Quad CPU's. Tough to read through all the 975x technical specs on Intel's site.
Dell is not about upgrading old machines they are about selling you a new one so they are no help.
Honestly, as long they keep providing the specs of the older machines I am good with that.
Upgrade your BIOS to the latest version and pickup a Q6600, Q6700, QX6600 or QX6600 CPU.
I added 2 GB of RAM for a total of 4 GB (XP sees only about 3.5GB of it), a Q6600 and 1 TB Drive for 250.00. Small investment without having to mod a thing. The only thing I can't squeeze any more performance out of is the on board disk SATA controller.
The only concern I had is the Q66xx CPU's can use up to 105 watts. The e6320 that was originally installed was a 65 watt processor. I used Intel's TAT (Intel Thermal Analysis Tool) Google to find and I maxed the Q6600 CPU to 100%. It ran warm with the stock Dell cooler but not HOT.
I optimized the machine for Pro Tools 8.04 with a Mbox Pro II Firewire interface and it rips right along.
I hope this helps,