The full montevina chipset is not going to be shipping until August. Apple may release something then (unlikely) or three months later, who knows. Apple will NOT be shipping anything with WiMax in the next 18 months, so if you're waiting for that, be prepared to wait a lot longer. As well, the integrated graphics in montevina CAN be a lot faster, but you have no idea what version of the chipset Apple will actually use, so discount earlier statements about "three times faster". DDR3 is also unlikely on the Macbook for a long time due to its present cost.
So...basically what will come out of the montevina upgrades are slightly faster processors and likely slightly faster graphics. If you're willing to wait for the bump, have fun, but if a machine that is shipping now will meet your needs, go ahead and go for it.