I haven't done video on a PC in over 6 years so I can't really weigh in what's what there, but if you are at all open to a different platform I can attest that the Final Cut Suite is a fantastic set of tools for editing and compressing HD video. Up until today I was using it to edit DVCPro 720p on a 6 year old PPC G5 2.5Ghz with 4GB Ram. That hardware surely had its limitations with exporting final edits (hours and hours of compression) but I could do ANYTHING else on my machine while it was doing it (except maybe edit more video).
Today I just got a brand new Quad Core Xeon Mac Pro because I killed the PPC, lol. I literally just finished installing FCP Suite and am anxious to see what render times are like for me now compared to last week - wahoo! Anyways good luck with your machine.
