Get the most powerful processor that you can afford. Nothing else effects Vegas performance than processor power.
Get at least 2Gb of RAM for each processor core.
I personally avoid ATI because of problems with the drivers – maybe it’s just me, but I always buy nVidia Geforce cards. Most recently a pair of GT520’s in one system and a GTS450 in another.
Steve Mann
MannMade Digital Video
http://www.mmdv.com