Frank I dont know how much of a hurry your in but if I was you I would get the system you want then install xp, (provided you have a legal copy and not an oem.)

Then I would wait untill windows 7 comes out late 2009, 2010 and install that OS at that time.

The main reason is you will eventually want to upgrade to win 7 anyway, as it will be much better than vista, and you will be kicking yourself if you pay good money for vista when win 7 becomes available.

As far as your hardware goes, I would personally go with some NVIDIA SLI technology. Right now they have 3 core, even quad core video.

Here is the stuff you need to be SLI ready:

http://www.slizone.com/object/slizone_ecosystem.html

I would go with (it is dual gpu)NVIDIA Geforce 9800 gx2
http://www.nvidia.com/object/geforce_9800gx2.html


but there are a lot of options out there; take your time and dont buy on impulse.

Id also go with a RAID