Nope, you'll keep everything.
First things first back everything up, there's an option somewhere on the ps3 to do this, and it'll back up all saves and such.
Then you get the ps3 firmware from the sony website and stick that on a memory stick in a folder called update in a folder called ps3
/ps3/update/whatever it's called.
Then you stick in the new one, format it, and it'll get you to run the update from the memory stick.
Then you use the restore option somewhere in the menus.
I can't see why you'd lose backwards compatibility either.
Edit:
http://uk.playstation.com/media/2Pv7AFEC/CECH-2102B_2103B-3.20_UKANZ%5BEN%5D.pdf
Found this, it should detail everything.
And not to insult anyone's intelligence, but it has to be a 2.5 inch SATA hard drive.