Depends on which HD 5800 you're talking about. Both the HD 5850 and HD 5870 are quite a bit faster, anywhere from 1.5-2x. If it is the HD 5830, then it is a toss up as they bounce back and forth. One big advantage the 550 Ti has over the HD 5830 is that it uses quite a bit less power than the HD 5830, Video Card Comparison - GPUReview.com
which would equate to a lot less heat and likely less fan noise. Comes down to which you would rather have.