x38/48 and p35 were specifically made for 45nm. Some 965's are compatible but that appears to be a long shot atm. I got an ip35 pro specifically planning to upgrade to a 45nm quad when they come out. Most mobos need a bios update for 45nm, however, so make sure that you have a spare cpu with which to flash the bios if you go the e8400 route. the e6750 is newer stepping than e6400/e6600, but you also need better ram for it or the e8400 to get a decent oc. Make sure to get 4-4-4-12 ddr 800 or maybe even some ddr 1066. I haven't checked the 1066 prices lately but 4-4-4-12 ddr 800 was hovering around $40AR for good name brand 2x1gb last time I checked. As long as you're buying new ram instead of inheriting some cheapie ddr 533 or something you'll be fine with the e6750/e8400. I would recommend waiting until march if you can, however, since it is likely that the e8400 street price will drop quickly when the 45 nm quads come out.
edit: I don't think that x38/48 mobos need a bios flash, but start at $200 and are more than most people want to spend. IP 35 pro is nice at $150 and p35 DS3L is really nice at about $100 right now.
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