Here's what i find good spec's for HD video are...
-Quad-core idealy, 2.4Ghz works well, but overclock for better results (Q6600)
-3GB or more for vista, 2GB for XP you might just be okay, but to be honest RAM is cheap, get as much as your motherboard will take.
-Video card i'd recommend a 8600GT 256mb or more, i get great results from a 8600GT at 512mb.
-At least 2 hardrives, 1 for your OS (can be small) and at least one for your video files, perhaps one for video clips, one for rendered movies. 7200rpm minimum. Setting up in RAID (i think 0 or 5) is best.
- Try and get a monitor that will display at 1080p.
-PSU, id say 450watt or more if you want to be sure all your hardware will be properly fed.
- Best to get a fast SATA drive that will do everything, up to you if you want blueray, but lightscribe is nice to have.
Software...
You mention you have windows stuff already, that's fine, there's no real reason to switch to mac unless you want to work with Final Cut Pro.
I use Sony Vegas Pro, and think it's better than both Adobe Premiere Pro and Final Cut Pro, Sony have just released Vegas 9 which looks just as good as 8 (edition im using).
To be honest it's up to you, but try and test them, as you're going to want a suite you're comfortable with considering you will spend so much time on it (a lot more time than you will predict).
Vegas just seems to make most sense, and works faster than anything else i've tried.
Results are going to vary, but a system like that ^ (pretty much what i'm running) is going to be fine in most cases, and invest in a better cooler, and overclock that Q6600 for more intensive work.