If your running an opteron 146, upgrading to a socket 939 X2 4600+ will be the best bang for your buck. Anything else will require moving to DDR2 (200 for 2 GB of performance DDR2), and 120-250 bucks for a Core 2 Motherboard would be the immediate price. Then throw in a Core 2 E6300 for another 200, and you already spend over 600 bucks. You would spend less for an AM2 mobo, and still need DDR2. Despite this, the upgrade from 939 to AM2 is really mediocre at best and I see no point.
Id say get an X2 4600+ for your socket 939 motherboard, this will provide a huge multi-tasking boost and still be a very good system for under 300 dollars total. Then in another year, once Conroe prices have dropped from their current state, and the motherboards start to mature for conroe you might make the jump to DDR2 and a conroe mobo, or even DDR3 in a year.