If money isn't an issue and/or your gaming resolution depends on it, then sure do the step-up. But a word of warning: EVGA and so on go based off of their MSRP prices. Rumors are that the next GTX280 can cost $600+! So if you buy a 9800GTX now for $300, be prepared to spend well more than $300 to step-up.
Also, FWIW, people have been rumoring that the 4870 can be +20-30% faster than the 9800GTX. Granted, those are rumors, and the card will be priced at $350, but either case, I'd wait the extra month because whether the new cards are better or not, the currently existing cards will go down in price and you'll win either way.