Dell Precision 390 , Which CPUs work for an upgrade?

We have two Dell Precision 390. One with the Conroe E6300, and one with the Pentium 4 631, Cedar Mill variety.

We would like to upgrade the P4 to something better.

We have a Q6600 CPU lying here, and a ancient Pentium D 830 dual core CPU, but it's gotta be better performer than a single core  P4.

Are the motherboards the same in these two 390s? I'm thinking our E6300 could be upgraded to a Q6600, but the 390 with the P4 I'm not sure about. They both are socket 775. We already have the correct heatpipe heatsink needed for the hotter CPUs.

Thanks