trhe E7400 will give you at 333FSB already 3500Mhz, probably at stock volts, if your board can do 400FSB, 4ghz is within reach... I would buy a tray model mate and get a decent cooler for it ( do not run it with a boxed cooler )

And plz make sure your board supports the newer 45nm CPU's, eg Asus P5B Dlx had some issues when OC'ing them, latest beta bios fixed that..