Alan's right (again !) -

PPro 1.5.1 with Cineform Aspect HD is the way to go if you want to edit HDV because it takes the strain off your PC. Bottom line is you can buy a bog standard 3.2GHz hyperthreader from PC World (as I did), add a SATA video drive (as I also did) and you can edit, scrub and playback HD as if it was SD - its effortless. It will even play four superimposed layers of HD video from the timeline (probably more, but I got bored adding layers).
Other NLEs, such as Sony Vegas and Pinnacle Liquid Edition will edit HDV natively without the intermediate codec conversion stage that Aspect HD requires, but you need a lot of extra processor power to edit MPEG2 on the fly - preferably dual processors or dual cores (or both).

GG