I built a new computer back in November for video editing. While I cannot cite them to you, many of the articles I read listed three main areas to consider for video editing: (1) CPU (they recommended the E6600 because of the larger cache), (2) memory (most articles recommended 2 gig ... it was impossible to find objective comparisons of 2 gig vs 4 gig), and (3) speed and number of hard drives (such as 1 disk for progs and 1 for data or, ideally, 1 for progs, 1 for source files and 1 for output of files).
If you had to choose between the E6600 & 2 gig memory or E6300 & 4 gig, I would recommend the E6600. Note that if you go with 4 gig of memory a 32 bit OS will only be able to use 3.5 gig ... you can find the full explanation for this if you do a search.
Everything I read said that the video card will not have an impact on the speed of video editing. At first I thought that ATI was marketing the X1___ series as speeding up transcoding (called AVIVO), but found out later that the hardware acceleration only applies if you are using the ATI transcoder utility. It will do nothing for video editing software.
As to the hard drives, not only does it help to have more drives, but in setting them up a lot of the resources I checked recommended separate partitions for the Windows virtual memory and, if using Adobe, the Adobe scratch disk. For example, on my system I have the OS on the Raid 0, the source files on one hard disk (which also has a partition for the Windows virtual memory) and output to another hard drive (which has another partition for the Adobe scratch disk).
Finally, I do not know what software you are using or planning to use, but I actually found this to have a huge impact despite the system I eventually built. If you are looking at the consumer packages, I would highly recommend Adobe Premier Elements 3.0. I have Pinnacle Studio 10 and ULead 9 as well, but find Adobe to be much faster and less prone to crashes. (All of this applies to Win XP pro ... I do not know how they would do under Vista)