The only one i know for sure that would work is a T3400, but that's not much of an upgrade, as it's only 160MHz faster than what you've got. As long as the BIOS supports it (and very likely would if you're running the lastest version), i'm very confident a Core 2 Duo T5850 should work in that system, and is the fastest Socket P 667FSB processor you can get. If you're interested, i'd check eBay, as you can usually score chips like that for $15 or so.