I do a fair bit of video editing, both SD and HD, I do use Sony Vegas and not Adobe lets see how much processing power we can take to run our software premier, but I do like After effects mind you.
Anyway, memory, as much memory as you can get, lots of hard drive space, because if you are going to go into Hd, you will need it.
My set up is AMD Phenom II quad core 3.5 CPU with 8GB of ram. My video card is a bit old a Nvidia 9400GT, but it does the job as the video card is not a major thing.
I can edit HD up to about 3 or 4 tracks depending on the effects in use.
A lot of people will say go for a Intel CPU, I like AMD.
Monitor wise I got a Hanns-G HH221, fully HD, it was just over the £100, but the latest model is under the ton now
Adrian
Desktop machine now powered by windows 7 pro 64bit , laptop by ubuntu
On ADSL24 using C&W network.