It states that you can buy a 537 with a Core 2 Quad, but when I asked their tech support they stated that they did't support any processor upgrades.
Dell tech support are a bunch of idiots and they will never tell you if you could upgrade because they rather you buy a new system.
CPU Support Specs
Intel® Celeron Dual Core
Intel® Pentium® Dual Core
Intel® Core™2 Duo
Intel® Core™2 Quad
Intel® G41 Chipset
The G41 chipset will support any of the core 2 quads except the QX9770 other than that you will be ablt to run any core 2 processor in it but definitely update to the newest bios they have on the dell website befor putting new CPU in.