I went ahead and snagged the HD cable for my DPL (digital light projector), and there is definitely a difference between normal (480i), and 1080i. 480p also looks nicer than 480i, but not as nice as 1080i. Where you see the difference the most is in the scenery. Tooling around the tracks, I noticed a lot of detail I had missed before. My cables were about $40.00, plus the component cable for the PS2. I bought the Sony cable for $25. So, including tax, shipping, etc, I was just over $70 for this. I guess it was worth it. I wouldn't spend too much on the whole HD thing though, because the difference, while nice, isn't enough to justify spending too much on it.
Also, much to my dismay, and like so many other great features, HD output simply doesn't work at all in Two Player Battle Mode (TPBM). The one place I was really hoping it would work, because all of us have a harder time seeing the courses in TPBM. I thought the extra resolution might help. Neither 480p nor 1080i works in TPBM. So disappointing!