With a 14 year old computer, upgrade options are pretty cheap.
This has a Core 2 Duo E7500, running at 2.93ghz, with 1066fsb and 3mb l2 cache.
According to the technical guidebook, the max cpu is a Core 2 Quad Q9650. It's 3ghz with 1333fsb and 12mb l2 cache.
* fsb is front side bus, the connection speed between cpu ram and other components on the motherboard.
* l2 cache is memory on the cpu that is significantly faster than system ram. More cache means the cpu can store more info for processing without fetching from ram with a delay. Cache usage is measured in hit rate, the percent of requests that are fetched from cache vs ram.
A Q9650 can be found for $35 on ebay.
Memory
There are only 2 ram slots, and the manual says it can support up to 4gb. I tried a 4gb stick and it happened to work, so I stuck another one in and it ran with 8gb.
I ordered the best 2x4gb ddr3 I could find on ebay, for $10.
Overclocking
The Dell motherboard has zero overclocking options, but you can hack the chip with a piece of electrical tape. This is called the BSEL mod, and tricks the chip into using a 1333 fsb. The cpu has a fixed multiplier on the fsb to set the final clock rate.
The base fsb is 266mhz, and the memory speed is x4 to get 1066. This cpu has a multiplier of 11 to get 2.93ghz. If the fsb is set to 333 with the BSEL mod, the cpu runs at 3.66ghz, a 22% improvement.
I ordered an upgraded heatsink for another Dell to do this without worrying about increased heat. You need the type with heatpipes and a bigger shroud, available for under $20.
Xeon swap
Some xeons of this generation are LGA775 and can drop into the slot, but most are LGA771. To use an LGA771 chip, you need to chop some tabs off the slot in the motherboard and apply a special sticker that bridges pads on the bottom. There are cpus on ebay for under $20 that are soldered and notched to fit an LGA775 motherboard.
The bigger concern is compatibility. The Dell bios is missing microcode for some xeon chips. Earlier C0 steppings are reported to work, but E0 need a modded bios.
I cannot find a copy of the bios mod anywhere. There are links in forums, but they're all down and there are some reports of malware. There's a guy with a donation page that might be able to help out, but I'm less inclined to go down this path.
Some chips I'm looking at.
X5470 - 3.33ghz, highest option at 1333fsb. E0 stepping only, so needs bios mod.
X5460 - 3.167ghz, C0 stepping probably will work.
E5460 - 3ghz, C0 stepping should work. 80W low power version.
The X5470 and X5460 are 120W TDP chips, which is pretty hot. A Q9650 is 95W, which is the max this system was designed for. The E5460 is appealing for better efficiency.
Conclusion
When the new heatsink gets in, I'll try the BSEL mod on the E7500. If I want to upgrade, I'll probably get the Q9650 because it has an integrated gpu, and to run a xeon I'd have to get an external card. The performance is all around the same, but the overclocked E7500 might be the best for single core performance if the 3M l2 cache doesn't hold it back.