Well, AMD is simply out of the question at this point in time. And for future games and applications, video encoding and other digital lifestyle needs, a quad core will probably "last the longest". I'd go for the high end Intel Q6850.
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115026
256kb Level 1 cache and 8mb of Level 2 cache, good stuff.
You could always go crazy and build a dual xeon machine, but I rekon a single quad will be the best bet.
Remember Intel has speedstep, and VIsta supports it, it doesn't have to run at 3.0Ghz 100% of the time.
I know it is expensive but it is what it is, top of the line.
If not the top of the line insane extreme edition...then any of these will do.
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=50001157+40000343+1050727216&name=Core+2+Quad
There isn't much difference between them, they are all the same architecture, just different speeds.
Bottom line, go Intel, go Quad. AMD is alright, but they are...subpar.