Another reason to get FW instead of USB (or better yet, a dual FW/USB enclosure), is that with FW, you can daisy chain your drives... and you can't do that with USB.
I'm in the middle of editing a 2 hour documentary using FCP 4.5 HD, and I'm doing it on a G4 iBook! No problems at all. I'm using a Seagate 300GB external drive in a Metal Gear FW/USB enclosure. Make sure the drive has at least 8mb cache (16mb is better)... and 7200RPM (this is almost standard on 3.5" drives these days).
And I'm sure you know, the key to editing is having a good monitor (I bought the 20.1" Dell 2001FP). However, what people sometimes forget, is that editing involves SOUND as well as PICTURE, so I'd pay attention to your speakers. The built in speakers on all laptops are garbage. They are worse than the speakers on most TVs, not to mention any kind of fancy system like surround-sound etc., that most folks hook their TVs to nowadays... if your work is meant to be seen on a TV, you're really handicapped editing the audio using the lappy speakers. Personally, I've sprung for studio speakers which I connect to my iBook (M-Audio Bx8a, self-powered, got a good deal on them through Amazon). You may also want to get an external soundcard that's higher quality than what you get out of your laptop, and hook your speakers through that, but if you're on a budget, you can skip that step. I understand you're on a budget, so maybe at least hook up your stereo to your MBP (I'm assuming you have a stereo!), that way you get better sound than from the crappy built in speakers. Good luck!
