AMD motherboards and CPUS are a cost effective way to build a nice computer. Their FX-series and the newer A10s are very nice. Sure theyre not as potent as a Intel in raw processing power...but they are top-notch for multitasking and have PLENTY of oomph for gaming. They surely wont be your bottleneck.
If you put a new mobo and CPU in a OEM case like your dell...you'll want to explore the options of case cooling. See if you have any extra places to mount fans. look at the fans you already have...perhaps youll need to upgrade to something that can push more CFM. Keep that in mind. Putting high end hardware in a OEM case is not a bad idea if you can keep it cool and keep the air flowing. very important.
Your new mobo should have several places to install fan plugs...and most Mobos will give you complete control over the fans and the way they function. My ASUS Gryphon z87 has lots of BIOS options, and it comes with a nifty program that is run inside windows that allows you to tune your fans how you like. It also has a auto-setup for your fans to see how well your case flows, and auto-adjusts the fans accordingly. Works well for the most part.