Unless you can save up enough for the new Kepler in a reasonable amount of time then it might be worth getting the GTX 570. If you are buying from Newegg then they do have purchase options like bill me later and prefered account. If you were to set up a Newegg prefered account then you can take up to 6 months with no interest to pay off the amount if you purchase a minimum $250 , if you spend $500 then you can take up to 1 year with no interest to pay and in either case you can pay the full amount at any time.
So you do have some options and it's going to be up to you to decide. You can also drop down in resolution to extend the 450 for another few months to save more money for the Kepler. The 570 is still a good card and will be a good card for some time to come.