Yeah the new iMacs have only FW800 ports.
So for my MOTU Ultralite (which is FW400 of course) I had to get a 400>800 cable as recommended on the MOTU website.
I agree you definitely don't need a good audio interface if you just use VSTs. In fact if you just use VSTs you don't need any hardware at all. Maybe just a nice mic + pre (w/ spdif) and a controller keyboard and you will sound great.
But I couldn't live without analogue basslines..so good A/Ds are necessary.
A cool thing about the RME 800 though is that you can use it to sum. So you can make your track 100% ITB with VSTs and then sum it internally with the RME to get a bit of that mystical separation.. kind of like using a Dangerous D-Box (albeit digital)..
The RME 800 really is the best around, for ITB or hardware-based setups, if you have the budget for it..