I was thinking Micro-ATX, possibly even re-using my existing case (although I'm likely to get a new one, although I'll be aiming for no sodding LEDs and side-windows if I can possibly avoid them.
Presumably with a mini-stx board, the size of the graphics card is a consideration?
Trying to think about what else I'll need - Keyboard, mouse, SATA DVD drive, storage HDD and monitors are all being kept from the existing system. Might try to move my Windows key across to the new system, although I can't remember what level of legit it is (it's quite legit, but might be OEM).
I've got a wi-fi card which plugs into a PCI Express slot on my motherboard, which I presume will still work, and an ageing Lexicon Lambda Studio USB Audio Interface I used for recording with, which I fully expect to mess about with for a few minutes trying to sort out ASIO issues and a high-pitched whine which won't go away, before punting it in favour of something more contemporary.
Things currently semi-permanently connected to my pc: Printer, keyboard, mouse, audio interface, external drive, joystick, SD card reader. I'll want USB ports. Lots of USB ports.