So this guy figured out that you should be able to take out the Core Solo processor from a Mac Mini and replace it with a Core Duo (ie, it’s socket-based — not soldered onto the board.) However, here’s the thing. So say you buy a Mac Mini Core Solo at $599 and then you decide that you want to replace it with a Core Duo. Just the Core Duo chip retails at $400 — and the difference from Apple between the Solo and Duo is $200, not to mention the DVD burner and bigger hard drive that’s been thrown in. Do the math.
To be fair Cyrus, the $400 Core Duo is the 2GHz version. the 1.66GHz Core duo is $253. Here's a link. http://www.newegg.com/Product/Product.asp?Item=N82E16819111180
^ What he said.
Still, I think it's not worth it to do the add-on yourself.
For me it's not really about cost-effectiveness as it is raw performance.
It's also about knowing that in 3 years--the day after your Applecare is history, you'll have the option of upgrading your Mini's dual core and throwing in a 7200 rpm hard drive for about $300. That's a nice perk for those of us who keep our machines for five years.