You won't get an AT case or components if you're buying new now. They're an old standard, not made for some 5 or 6 years past.
The only comment I would make about cases, is try and manage with a standard size - the mini cases are all well and good, but are a tad "crammed - so makeing cooling more problematical, and upgrading difficult.
With a standard sized tower, you can use any motherboard you want, as there will be space for any size. It also means no buying mistakes over sizes, as almost anything you buy will fit into the case.