20% is hardly a huge margin. As for me, its going to have to be at least twice as fast as my current processor for me to upgrade.
This depends on what you have right now. If you have a BTO 15" 3.06ghz C2D with 8GB RAM and Intel SSD, then the margin for improvement is small.
If you have a 2.4ghz C2D with 2GB RAM and 5400RPM, then there are real performance gains to be had.
In single-threaded applications and everyday usage, clock for clock, the benefit of Arrandale over C2D is about 10-25%. In multi-threaded applications, the clock for clock benefit is about 30-45%. So, if you have that 3.06ghz, the best you can hope for is a 2.66ghz i7 which should put you near the equivalent performance of a 3.33ghz C2D. That's not a big increase. If you have that 2.4ghz, then you could get that same 2.66 i7, and the jump in performance from 2.4 to 3.33 will be more worthwhile.
Also, as we all know, operating systems and applications are becoming more multi-threaded. The system you buy today may get you a 30% performance increase right now, but the same system 12 or 18 months from now will be 40-50% faster than your old laptop. Of course, the argument could be made that you should wait 12 or 18 months until upgrading, but I won't open that can of worms here. If you don't need an upgrade today, then don't bother. Upgrade when you need to upgrade... otherwise you could be playing the waiting game forever.
I have a 3 year old MBP with a 2.33ghz Merom maxed out with 3GB of RAM. I use VMware 100% of the time, so I definitely need the ugprade. I will get a 2.66ghz i7 MBP as soon as they become available.