You can buy an E6600 for less than $400 on its own, and the 'included' processor is somewhere around $150. Don't see how the $250 price difference equates to almost $500 in upgrade price. Just goes to show that you should build your own stuff. Those are some seriously high upgrade prices, they're akin to Apple's upgrade prices. But, if you aren't gonna build your own, just get the stock one. It should run vista fine.
The Core 2 Duo's do shit all over AMD, sadly. I've bought nothing but AMD since the original Athlon, but my next upgrade will be a Core 2 Duo. The E6600 beats every single AMD processor on the market in every benchmark that I've seen (audio, video, gaming, regular computer stuff, etc, etc). AMD slashed their prices in reaction, but the Intel's are still a better bang for the buck. Yeah, Intel and value in the same sentence. Haven't said that since... ever.