Not so much an After Effects issue as a hardware one

I couldn't find a better topic to post this thread in so I apologise if it's not welcome here.

My problem is this:
I am looking to upgrade my Macbook to a desktop, but I can't decide between the new iMac or a Mac Pro.

Now I work a lot with video editing, with such programs like After Effects (which is why I posted it here) but I'm also looking to develop my 3D skills.

Currently my Macbook is running with a 2Ghz Intel core 2 duo processor and I upgraded to 2gb ram about 6 months ago.
Already I've noticed it's starting to lag with 2gb and although I'm not looking for an immediate purchase, I just thought I'd collect some advice.

The obvious things to take into consideration are the costs and the specs. I know the iMac comes with 3.06 Ghz processor and 4bg ram which seems like a massive improvement over my current numbers. (i priced it up to about £1300)

However, the thing is, I've only had my Macbook for 14 months so what I really want to know is, is the iMac going to last?
It maxes out at 4gb (well the current model does anyway)
Then the mac pro, which maxes out at a crazy 32gb RAM seems pretty future proof, but obviously costs are sky high. I should mention I'm not looking to buy a 32gb ram mac pro right off the mark but it's nice to know i have that option in 5/6 years time..

Does anybody have experience with either of these two machines?
The Apple forums were pretty biased (bigger=better it seemed.)

Being a student i get ok discount but around 2 grand is still a heck of a lot of money for me.

Thanks in advance