With a little bit of shaving you can fit a 3.5" in. Quite tightly. The main problem is power - either a seperate psu with the cable leading into the side of the case (beside the Escape key is the best place) or a small cable with 2 floppy type connectors and one HD connector to replace the floppy one that's already in there.
One more thing though, the keyboard ribbon cable might get slightly damaged (the 3.5" has to sit on top of it and it gets creased) which could cause problems. Saying that, I had a 3.5" 2.4Gb hd in my 1200 using the A1200 PSU only and got about 2 years out of it before towering the whole thing.
I would strongly recommend getting a 2.5" if you can get it cheap enough.
--Edit--
Oh, one more thing.... Welcome! :pint: :-)
