The 300gtx are OEM cards, I assume you mean Fermi which is now called the 400 series, they should be out by the end of March and you will probably need to fight tooth and nail to get one before they are sold out.
Why do you need to upgrade, a 285 gtx is already a beast that can last you another year. What is your budget and screen resolution? Getting ATI's new 5850 or 5870 would be a slight upgrade for $300-400, maybe not worth it for that much. The 5970 would likely double your performance for $600 but unless you have a 2560x1600 screen it is a waste of money.