Jimmy: yes, it'll be a m4a file. This is just an aac file, as I said.
Media Player should (I think) be able to play it. If not, then it's Media Player being rubbish, not iTunes. aac files have been able to be played by most Nokia and Sony Ericsson phones in the past 5 years, nevermind your computer.
CheekyChick: Yes, iTunes uses 256kbps files and 7digital's are 320kbps. But iTunes' format is aac, not mp3. This gives better results anyway, so the end result is the same, or if anything, I'd put money on aac.