regarding aftermarket ram and stuff, i've always liked these guys:
http://macsales.com/

not an official endorsement, and i hope i'm not dissing a site sponsor or anything. i have no affiliation, but when i used to configure edit systems for my job, their merchandise, service, and pricing was always good. now i actually get paid to edit and i can let our i.t. guys worry about that stuff. good luck though.

i'm going through the process of converting from a.e. to motion. i still use a.e. from time to time, and if i was primarily a motion graphics guy, it would be the only thing i use. but when you get the hang of the slightly weirder interface, motion really comes in handy because of its integration with fcp. being able to send a clip from your fcp timeline to motion for effects work, and then return to fcp with the clip updated is really sweet. to do the same in aftereffects you need a 3rd party app.