hey buddy i'm too looking for upgrade solutions. i've been gathering info from all over this forum so here's my suggestion:---
Intel Core2Duo E8500, E8400, E8200 (newly launched)
These babies r based on the Penryn core (45nm fabrication process). I'll recommend them bcoz they r the top-of-the-line, serious revisions have been made and r an overclocker's delight (rock solid gaming performance)
They may xceed ur budget but u'll get huge returns