I'm keen to avoid having to buy a complete system every 18 months or so, when I feel like upgrading my system. Presently I have an EVGA setup (mb with 8800GTS 320mb video card), with an Intel Core 2 Duo 6400 4Mb.

I'm looking at a quad core option below about $400 US/CAD. This seems to be the best you can get for this kind of money:

Intel Core 2 Quad Q9450 Quad Core Processor LGA775 2.66GHZ Yorkfield 1333FSB 12MB

I have some questions:

1. What kind of performance gains should I expect by just upgrading the processor?
2. What's the 1333FSB, and will that make much difference compared with, err, a lower FSB ?
3. Is it easy to replace a CPU but retain the motherboard? I expect Vista will ring alarm bells like it did last time I made a far more minor change.
4. Is there any other Intel processor around my price mark, that would be better?

As always your advice is appreciated.

Thank you