Looks like not all motherboard screws will fit in the new case, this is manual
http://www.dell.com/downloads/global/products/optix/en/...
Back I&O panel (colored one, it covers spaces around USB connectors) would have to be carefully removed and moved to the new case.
No matter which case you decide to buy, you must save your old case - there is great possibility that you would have to use some wiring from the original case!
Now about new case. Here everything depends on your budget and your aesthetic preferences.
Go to newegg.com and look there, pick up what you like and post link here.
Meanwhile, we have to see if your PSU can be upgraded.