Edit: And researching some more, it seems that card (GTS 450) would require a PCI Express 2.0 interface, but my http://www.asus.com/product.aspx?P_ID=dFAMIWrZWGgB1GuJ&templete=2 has an http://en.wikipedia.org/wiki/Intel_P35 that can only do 1.1 so i wouldn't get the full power from that card with the current MB. Would the performance hit make it a waste of money?
First question, usually no. Just swap new CPU in and system should immediately recognize new one. It's usually the motherboard swap that would need a new OS reinstall...and even then, there are people savvy enough to provide guides on the web to even avoid that. Just make sure you've made any necessary BIOS updates for the motherboard in order to see the new chip before installing it....and of course, I hope this chip is compatible with the board.
The GTS 450 will run on any previous generation PCI-E interface...it's a backwards compatible thing here. As far as bottlenecking the card, I don't see it happening with a GTS 450 even on PCI-E 1.1, so should be fine...if there are any bottlenecks, it would be rather negligible.
Thanks in advance.
Should be fine in what you're planning to do so long as you have a Vista install disc. The rest of the drivers can be gotten off Asus's website as you've noted.