first off: any more details on what your style, setup, aims are, would be helpful.
2. I'd go for Intel but that is a fanboy and money discussion
The cores don't matter for now as Avenue only adresses one. Maybe the guys at Resolume HQ got something in petto? Are you planning to use that machine as a workstation as well? Then I'd say get as many cores as possible
3. As I just mentioned in a previous post on Matrox' TripleHead2Go the cards from nVidia do support higher resolutions on these units (3 x fullHD compared to 3 x 1280x1024). Most ATI cards however have four outputs of which three can be used simultaneously (eg. 2 x DVI Beamer and 1 x display port attached to a monitor). And there is even one fancy card with six display ports (look for "eyefinity 6 edition").
4. Don't know. I guess the difference won't be as serious as comparing HDD with SSD.
5. As Avenue is not 64bit it can't adress more than 2GB of RAM (I think it was 1.75Gb but who cares). However it does take advantage of more VRAM.
Of course there are many more details to consider. But I would need more details about your system etc. for those.
Geez! I hope this was all correct
it's late. Good night.