The iMacs have, basically, Montevina in them now. Technically they're not Montevina, but the specs are the same. If there is an update, it will likely be a silent one to true Montevina chips but no one will notice.
The next update for the iMac is Nahalem, which will not be out for some time. Unless you're prepared to wait at least six months for new technology, now is a great time to buy an iMac.
It could be as long as 9 months before they come up with another major update. Buy now and then upgrade in 3 years with an iMac that exceeds anything Montevina would ever do for you and sell your current iMac 3.06 at that time. It won't go down that much considering you will get 3 years of use out of the thing.