Lower end model here. Same here, I was umming and ahhing between the high end Macbook and the low end Macbook pro. In the end, I'm glad to have gone for the pro, having seen the side-by-side comparisons of the viewing angles - you do get a poorer screen on the MB.
But another £350 would've been rather difficult, couldn't justify it for the extra 0.13GHz and 256 of video memory. The other upgrades - RAM and HDD - I can do myself sometime down the line when it starts to feel "slow" compared to modern machines.
I think it'll last just fine. As a benchmark my 4.5-year-old 1.33 G4 still doesn't seem bad, it was only in the last year or so that it started to show its age when trying to play higher resolution videos. And that was the PowerPC platform, which I never thought was all that impressive in the first place.
The Intel stuff seems to stay useful for even longer. I have a 4 year old PC which is just about as fast as the 1 year old one I replaced it with!
HDD, RAM and Snow Leopard ought to be enough of a boost when the time comes.