Sorry it took so long to get back but things have been in turmoil around here.
The most important thing to keep in mind is that going with a desktop system is
very complicated and depends not only on the CPU and the amount of RAM but on
bus speed as well. Then there is the issue of compression and whether or not
the compression can match the quality level of your tape format, if you are
using tape at all. For example, if you are shooting BetaSP, then you will want
your desktop software/hardware combination to be capable of compressing at a
ratio of about 4:1.
There are other issues as well, much more than can be covered in this reply,
but bear in mind you can never have too much RAM and, regardless of the CPU, if
the bus speed is too slow to handle data rates required by video, then you
aren't going to have quality results.
Tim