Keep in mind there will be issues with reinstalling the Operating System. When you change the motherboard Windows will likely detect it as a new System and will no longer be Activated.
If so, you have a few options:
1) Purchase a new Operating System (Windows 7 x64 Premium OEM for around $100)
2) attempt to Reactivate through Microsoft telling them your motherboard was replaced but it's the same PC (probably will work)
When you replace the motherboard you mainly need to know:
1) the socket type for the CPU
2) the type of motherboard (BTX, ATX..)
Without more information I can't say anymore. If you're attempting to create a gaming PC also keep in mind that your CPU will limit how good of a Graphics card you can purchase (an older HD-4770 will max out an X2-4800+; a modern HD5870 will use on average about 25% of the maximum processing potential of an i7-860.)
It's usually not worth upgrading an older computer. I advise NOT changing the motherboard unless you are having problems.
Many Dell PC's have Power Supplies that are insufficient to power a high-end graphics card.
The Dell MB has died & I want to replace it with my old Intel D946GZIS.
I bought an APEVIA ATX-CB600W 600W ATX12V Power Supply on sale and am prepared to do the sheet metal work to drill &/or cut the back plane to match the D946GZIS I/O shield.
So, more specifically; my focus is to understand what needs to be addressed regarding the power switch connection - I had seen a blog entry indicating the power switch wiring in the Dell is non-standard.
Apologies for the ambiguity of my initial question.