It runs like an x1800xl because it's underclocked. The design was a compromise: HTPC users want something slim and quiet, so ATI's design team gave this thing a single-slot cooler and underclocked the poor thing. The good news is it's a good deal as is, yet can be overclocked very well ...you just might want to put a better cooler on it.
If you volunteer your comp for distributed computing projects Folding@Home will support this card soon (because of the R580 chip). The GFLOPS performance of a high end ATI is about 100GFLOPS, about 8x better than a single-core high end CPU.