The 5700 won't hold a candle to the 5900. Despite what some say, (SOD=Idiot) the 8x v. 4x question is not pivotal. The performance difference just isn't very big. This question really boils down to what you're willing to pay for performance. The 5700 is a good card, but the 5900 is better and costs more.
Where there's a will, there's a way-even when it comes to screwing up.