As others have said, the 2.1 for the cost/performance is not going to be worth it - plus you run into compatibility issues with the 533MHz FSB.
Go with the 2.0 Dothan - it is a ~25% increase in raw MHz, but you should see an even more significant boost with the 2mb L2 cache.
Of course, all of this said, I'd only make the upgrade if you need it. Often times people upgrade simply because they feel that they can. If you never find yourself saying "Damn, this 1.4 Banias processor really is a bottleneck - I'd get a lot more done and enjoy my computer more with a better processor" - then don't spend the money to upgrade (or wait another couple of months until Sonoma becomes even more mainstream and you will be able to get it cheaper).