I capture mostly on a dual g5, heres what i suggest, up the ram, osx loves ram and 256 won't cut it for general use unless you like to wait and watch the spinning beach ball
, the 1.8 g5 is more than enough for what you want to do, the issue here seems to be the disk space/read and write speed, I capture to a separate internal SATA 300g drive, capturing to the boot disc is a bad idea, its usually fragmented enough to affect performance in a situation like this, Tech tool pro or equivalent can do a good job of optimizing that though if you are limited to one drive.What firewire device is your friend using? is it a canopus box affair or straight from dv cam? some canon cameras had problems with the firewire connection, heres a article on the affected models http://docs.info.apple.com/article.html?artnum=61603
As for CPU prioritizing, OSX does that fairly well but you can open Activity Monitor application found in the utilities folder to observe the cpu load and disk activity, you can also kill all processes there that might affect the capture process.
Does your friend have the file vault security option turned on? that will slow disc access, you can find that under apple, system preferences, security
Also which version of imovie are you using?
Thats about all i can think of at the moment, hope that helps