Wow cool thread, wish I found it earlier.
I have a "E520" as well. In quotes because it actually was conceived to this earth as a Dimension 5100.
The 5100 was restricted to Pentium 4 only. Pentium D could be run unofficially as well and worked fine, but BIOS gives nag screen with "Alert! Incompatible CPU every time which is super annoying.
So after owning this Dimension 5100 for like, 8 years or something, I took my chances with a used E520.
The E520 was shipped to me I didn't pick it up. To my dismay the seller shipped it in a large box with zero protection.
There was some damage to the case but overall it survived remarkably well.
Worse news came after, as there were leaky caps all over the board near the VRM, and the PSU was making a awful electrical noise when the PC was running, which amazingly it did without any issue.
So, I fixed the capacitors on the board using my soldering skillz, actually I failed the first few times. During these times the motherboard actually ran WITHOUT vrm capacitors
and it worked.
But I fixed it properly after that and have been running it since.
Anyway, because the case of the E520 was knackered, I transplanted the insides into my 5100 case. I had to cut the rear I/O plate for it to fit.
Everything was succesful, and it is running to this day.
Specs: Q6600/4GB DDR2/HD7770
The PC is not getting used anymore though. I tried selling it once but there aren't any serious buyers, only people who are looking to pick it up for a tenner.
So should I do the QX6700 mod? I only have the stock aluminum (not heatpipes...) heatsink.