chosenbydesign7:
The way I look at it, your best bet is to buy a new case. If you buy a new ATX case that is of comparable quality to the BTX case that Dell gave you, you're going to spend $50.
AFter you spend that $50, you're all set. You can do all the motherboard replacements that you want for the next 10 or more years in the same case. You will save money in the end -- if you buy a new computer from Dell, you're paying for components that you already have -- a new power supply, hard drive, etc. You don't really need to replace those parts very often. When the next iteration of Intel processors come out, your cost to completely gut and rebuild your machine (possibly increasing the speed over Sandy Bridge by 50%) at a cost of:
motherboard = 180
Processor = 200
$380 for a complete upgrade -- what would $380 get you over at Dell? Not a whole hell of a lot...
My advice would be to drop $50 on a ATX case and $50 on a ATX PSU unless you know that you're going to die in the next 2 years...
I know none of this answers your question and I do sympathize with you; I myself write this on an older Dell desktop right now that I'll be replacing with my first new build when Sandy Bridge parts fall in price. I would just urge you to follow my lead on this -- I'm certain you won't regret it.