4GB DDR2 and DDR3 modules are difficult to obtain anywhere at a reasonable price.
I would recommend an Intel i7 920 setup. The processors are far superior to the Q9xxx series. A further advantage is that many 1366 motherboards (the ones that take i7 processors as opposed to 775 which take Q and E series processors) have 6 DDR3 (max 24gb) slots, which means you can get 6x 2GB which should be sufficient for a while and when 4GB DDR3 modules are cheaper (doubt it will be soon) then you can go with that. The price of an i7 setup is slightly more expensive than a Quad setup, but the performance is much better.
Vista 64 and XP 64 give very similar performance for HW. I would say go for the one you are more familiar with.
Another important thing you forgot to mention is which sample sets you plan to use? Do you use HW basic or advanced? If advanced you may want to upgrade the 24/96 to something with more outputs.