ericbrown,
My suggestion is that you learn which Dell computers used the Dxp061 motherboard, then download from dell.com the user's manual for that model. In the specifications will be the FSB speed of the motherboard and a list of the processors that were sold for use with that motherboard.
I find the world of the Core 2 Duo confusing as there are so many series with very similar designations and fit the same Socket. The e8400 CPU is a
1333 FSB and I'm reasonably certain the problem is that you need a Socket 775 CPU running with a FSB of
1066. The following is the Passmark searchable listing of CPU's and you might search "Intel Core 2" and see all the many, subtly different variations and their relative performance >
http://www.cpubenchmark.net/cpu_list.php
Then, googlize likely suspects, check for Socket 775 and see which is their FSB speed, matching to your original.
It may well be that the fastest CPU you can use will be a Core 2 Duo 6700 @2.66GHz, but, it's
possible that any Socket 775 CPU with a 1066 FSB will work. Examples are the 1066 FSB Dimension 9200 which used the Dxp061 and Dell Precision 390- motherboard model unknown- the base model of which was sold with the Core 2 Duo 6300 @1.86GHz (possibly your original?), but both these Dells were also sold with the excellent Intel Core 2 Quad Q6600 @ 2.4GHz. If the Q6600 is possible, with that 4-core CPU, 8MB cache, and higher clock speed, you could see an amazing increase in computing power. On the Passmark CPU list above, the Core 2 6300 1.86 is ranked #1005 (the bottom is about #1700) and the Q6600 is #427. Keep in mind the Q6600 takes more power, I think about double your original which = runs hotter + may need improved cooling.
More good news> reports of stable overclocking of the Q6600 to 3.0GHz.
Cheers,
BambiBoom
"Measure twice and saw once."