First of all doing any video editing in FCP is primarily something that taxes the CPU not the GPU. There isn't a whole lot of hardware accelerated stuff going on in FCP, and if you look at Premiere CS5 it only supports a very limited set of NVIDIA cards for hardware acceleration (which eliminates all the new AMD-based Macbook Pros). What really counts is CPU power, and of course a quad core i7 would be ideal.
However I just bought an i7 13" MBP (upgraded it to 8GB myself), and let me tell you that performance wise it is great. I do recommend having an external monitor for editing, but otherwise this setup is more than enough, and the i5 will be fine as well for iMovie, Final Cut etc.
I have a couple of Core2Quad 3Ghz workstations (Hackintoshes) I use for grunt work in editing and After Effects work, and this little 13" runs circles around them. It scored around 50% higher in Geekbench, and in my unscientific tests rendering out the same clips from After Effects with a lot of effects, distortions and stuff going on, it varied from being 35-110% faster. Which blew my mind. The same was true in a couple of rendering tests I did in Final Cut, and the Core2Quad is fast enough for editing as is.
So take it from somebody talking from experience, you'll be more than fine.
EDIT: clarification