Yeah clock speed doesn't matter as much any more, like said the new core2duos can do more work per GHz than the older pentium 4's because their architecture is more efficient. A 1.86Ghz core2duo is faster than a 3.0 Ghz pentium D.
Uhh, I doubt it would cost more then a house. Try around $2000 total? Probably less? The Quad-Core Core 2 Duo is a true quad-core (AKA the Kentsfield.) Try wikipedia?
they aren't officially released at 3GHz unless you're going the server route. The fastest officially available is 2.93 and it costs $1,000 ehh... I got the 2.13GHz e6400 and over clocked it to 3.2GHz w/ an undervolt to 1.264V I'm happy enough.
