So its the Case that you want to keep?? Well that is Okay. I had a uATX Dell dimension case with a DELL motherboard (I mentioned before Intel (OEM) makes for DELL and MSI made a motherboard for DELL lately) Of course it can be swapped out with regular motherboards.
**MAKE SURE YOUR I/O (input/output) panel in the Back of the case is COMPATIBLE with Retail motherboards made bt Abit..Asus...Epox etc...(or look for one to place in there could take some work, but it can be done)**
ALSO the POWER SWITCH is DELL propietory, so if you plug it into a Abit motherboard it will NOT turn on the system. This can be easily fixed by removing the bracket DELL puts in front of the case with a "generic " HDD LED.. POWER LED... POWER SWITCH (to turn on the system) even with the DELL power supply, I mean come on, a power supply is a power supply. So there are MANY options to upgrade with the case in the future.
UNLESS it is a FLEX ATX style where PCI/ISA/AGP slot is Sideways instead of up and down in retail boards.
Sure DELL has a neutered BIOS which gives you limited options, remember DELL does not want you to overclock nor change system setting (even if you are a Pro)
Good Luck.