well in general, you don't want your system disk to be the one you store your media on. so if you don't have a second internal drive, or if you don't want to get one, then yes, an external firewire (fw 800 is ideal) drive would be good. i'd recommend a second internal, unless you have any desire for the media/drive to be portable... you really shouldn't see a problem running off any modern external drive. of course, if you're going to work with hd or some other high end stuff that requires more bandwidth, you'll have to upgrade...
for fcp and ram, i haven't noticed any real problems with my dual g5 with 1.5 gb. i would say that if you plan to use motion, or if you use fcp while other stuff is open, it would be very beneficial to throw another gb or two in there. i'm not 100% sure, but the ram amount you have may affect the ability to do real time effects, etc... though i think that the graphics card is a big part of that, as with motion.
for after effects and other such apps, ram is a huge deal.
