Just type "editing pc" or "video editing" in the search box and you will find a lot of the same open questions, with answers.
Think every more experienced videographer will tell you a different story about which type of editing software you should use, it all depends on what you want to achieve and what you feel comfortable with. I f.i. have tried working with vegas, but as an adobe fan I couldn't get used to the vegas interface and different workflow. You could try to download some trialsoftware to give it a go and see what fits you best.
For pc it also depends what your expectations are, I have an older 3,2 single processor pc and because I didn't make the switch to hdv yet my current setup workes just fine for me, I prefer stability over speed at this moment because I make a livin out of it.
If you edit hdv get a fast pc, core duo processor, 2 gigs of memory and lots of harddisk space, one harddrive for your programs and windows and one or more drives for your footage. Graphics card should have enough memory available. (256mb min) Especially if you want to work with vista and see all the beautifull (and to my opinion useless) 3d tricks it can do. Assure you have enough usb2 and firewireports and that you get very good cooling, many entry pc's get damaged hardware due to insufficient cooling.
I work with a tweaked xp pro version and expect to use it for at least 2 more years, I don't have any firewall, virusscanners or internetconnection on my editing pc (windows update? whats that?

) and that's the way it should be. Because the pc is dedicated to editing only I don't need security patches which sometimes can do more bad then good.