And with what impact to everyday performance?

The other truism of computing is that improvements to peak performance have little effect on everyday computing (unless you’re always doing computationally expensive work like gaming or motion graphics).

That’s not really a fair comparison. Processors haven’t been an apples to apples comparison for years (pun intended). At least not if you’re just using frequency. The new Xeons, for instance, run at a slightly slower frequency than the previous generation, but have much larger caches, use less energy and ultimately run applications substantially faster. Furthermore, the new systems use a much more advanced video card, which will be accessible for general processing when Snow Leopard comes out.

That being said, I’d love to see them expand the upper limit of the 15″ RAM capacity. The new 17″ Macbooks can handle 8GB, so I’d expect that from the next generation of 15″.