I like the Optiplex GX mini tower case and I have a literal stack (five of them) of the GX520 model. What was the best specification model to use that case?
I figure the easiest upgrade would be a board from the same product line since the non-standard Dell front panel connection will plug right in. Power supply is easy since it's standard ATX, 24+4 pin with standard ATX supply mounting screw pattern.
If there was another line of Dell BTX systems that have the same front panel connection, and went to higher specifications, that info would also be interesting to know.
The newest BTX boards I've found used a LGA 1156 CPU with DDR3 RAM and up to i7 CPUs so the form factor isn't quite dead yet.
The cases are proprietary and the front panel uses 20 pin ribbon cable that isn't documented. The 520 is the model that is the LEAST upgrade-able. A 620 motherboard would be the only upgrade.
These are LGA 775 systems with max upgrade being 2.8 ghz Pentium D 900 series aka 915.