There's just a few sizes of motherboard. I did a quick look at Dell's page for the e521 and whereas they give a nice 3-d rotating view of the 'puter, they don't give a lot of tech specs.
Incidentally, your case won't know or care if you switched to Intel instead of AMD. By changing the mainboard and cpu, you're gonna lose any warranty support you might still have with Dell anyway.
It looks like mATX to me, but you can just open up the box and measure to be positive. I've had the embarrassing problem of believing what the manufacturer wrote about my case and buying a full-size mobo only to discover it's micro not regular.
Reusing the e521 case should be just fine. A heads-up is the "I/O shield" of your new motherboard may not match up with the old one, and in a lot of proprietary cases the I/O shield isn't made to pop out easily. I've used tin snips to make way for a new one.
I don't see any reason you can't use the same DDR2 you have currently. I mean assuming you choose your new motherboard sensibly (obviously if it's DDR3 then . . .)
I want to upgrade my current AMD Athlon CPU to an AMD Phenom, but my current motherboard isn't compatible with it. Which motherboards can I get that are compatible with AMD Phenom (socket AM2+) and that will fit inside the case? I assume that my current memory sticks will not be compatible with the new processor. Thanks.