You're most likely going to be limited to Conroe processors with a 266 Mhz FSB/1066 quad pumped FSB, like the E6400, E6420, E6600, E6700, and X6800. That means you'll want to buy a used E6700 from fleabay, craigslist, or maybe
here, although newegg is still selling the E6600 for $230. You should be able to buy an E6700 used for considerably less, though.
edit: I didn't your your link. It says you can use any of the processors I mentioned, plus any of the E4x00 series. So, you could grab an E4600 for $120, or an E4700 for $140, and get a fairly significant increase in speed.