To run FCP smoothly, yes, 4GB total RAM for sure. If you're editing HD material, you really want to look at 8GB RAM. FCP will use up to 4GB of RAM by itself. The OS will use up to 4GB of RAM by itself. Thus, the 8GB RAM minimum recommendation. HD material takes up much more CPU time, and thus the more RAM you have the smoother and easier (and more stable) FCP will run.
2GB min in the FCP tech specs is marketing hype. Yes, FCP will run with only 2GB of "available RAM", for very short SD projects. But you get into HD, and if you don't have 8GB RAM, don't hold your breath for the fastest, most stable system.
As for hard drives, if you can get a 1TB, get it. It'll have faster read/write/seek times than a smaller drive. The larger the drive, the faster it spins, the faster it's heads fly, the faster it's controller board runs, the more of the FW800 data pipe it takes advantage of.
FW800 for some HD formats, some HD formats (codecs) will bog down a FW800 connection. What flavor of HD would you be editing? I.E. what camera is it coming from?