I'm thinking about the same move myself, sell current hardware now while I can get a decent price for it and go i7 860 with a cheap Biostar TP55 .
Anand had a good review of cpu here, shouldn't be any prob to hit 4ghz's with aftermarket air,
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=3641
But, again, it seems like most people are NOT getting 4.0 with an 860.