The hardest part with these drives is getting them crammed in the case ;P The cables are short and sometimes hard to get plugged in then folded down enough to fit snugly into the rather small case...
I've only made 3.5" as I'm not too worried about size. if I remember right, 2.5" drives actually don't have the molex power addaptor, so that might make things easier(one less cable to worry about)
Just look into some cases. See what size drive you want. Most cases now will work with 300GB drives, but of course you should double check that.
If you're wondering about interfaces, Firewire is the best. It's about 15-20% faster than USB 2.0. Now, there's nothing wrong with USB 2.0; one of my drives uses this. However, all in all, I have better luck with my firewire drives. Less problems and such.