I responded to these questions before as others here have already. Read those yet? Any help??
NO ONE knows what and when Apple will release a *new* 8-core (You could always try to email Steve Jobs and ask him). And users don't know when certain applications will go multi-threaded. Do you think they will all suddenly be multi come January? Or maybe later in 2008? 2009? Now, how much more video work could you do on your new 4-core system than your current machine in that time? Also, remember that apps can become inefficient if you multi-thread them, so that is not the end-all for every program. And *again*, Macs have a high resale value, so you could sell it soon.
But if I were in your shoes right now, I would really look at rendering tests on existing 4-core vs. 8-core and see what the difference is for the apps you use (especially with multi-threaded apps.) I think you may better find answers to this by Googling them. Try here for instance:
http://www.barefeats.com/octopro1.html
Is the cost vs. speed benefit worth it? That is something you (and your gf) will have to answer.