A good point, forgot about the PSU. Even so, I would want to get a 4850 for its price and performance and look to getting a better PSU that can cope - depending on the PSU you get it also means you have room for further upgrades as time goes on.
For example the XFX 650w PSU is $65 after rebate (I am so jealous of American prices!!). Obviously that with the card means $25 over budget, but it is a great PSU which will last for many years and be good to put in a whole new build in the future.
But, taking PSU in to account, assuming it is very weak and upgrading is not an option, the 5570 would be the best bet.