About a year ago I upgraded from the the 400 to a sonnet 1ghz. At that time it was a 250 dollar upgrade and it gave my computer a few years of extra life as the 400 won’t support many of today’s software upgrades. It was more than worth it to me and cut my compression times in half, not to mention being able to take advantage of realtime preview in FCP.
If you get the upgrade and decide in a year to get a new computer you’ll now have a less outdated backup system. I’ve never had any compatiblity issues either. I just popped it in the machine and it worked and continues to work. Your computer my be able to handle a dual proccesor as well, but not all 400 models can. You may want to look into that as well.
I don’t think you can go wrong unless you can afford to get a g5.