With the release of Vista, I would strongly advise you to hang on for at least 6 months if you can. Prices will continue to fall, and any bugs in Vista will hopefully be sorted by then.
In the meantime, read a few computer review magazines and on-line sites - see what you can get for your money, the minimum standard spec and so on.
Mac owners love their Macs! I know very little about them, except they are more expensive than pcs, there is less compatible software available BUT they are better for the sort of video editing you want to do.
Also, they work differently to pcs - so it will be a whole new learning curve for you. Having said that - they are far simpler to use than pcs.
You will be investing a lot of money whichever you decide. So take your time.