I don't know about how they would fare against a phenom. Some of the old opterons are good chips. That said, two might do good, but I don't think you would be getting your money's worth. Figure $150 bucks for a new one if you can't find a used one. So 2 of them for 300 bucks, you could realistically have a new AM2+ motherboard, 2 gb of ddr2 1066 memory, and a quad for under that amount. So at that, I don't think I could recommend the dual opteron dual core idea. Put it this way, if you upgrade your old stuff, you are dumping money into an old design. If you go with newer stuff, at least it's not outdated as fast. It's up to you, but as it gets older it may get more expensive.
Yeah, can understand. I know I got into that deal of do I try to upgrade old stuff or get all new, and it's really getting to where unless you have something old that's relatively quick and is loaded with ram, it's almost better to upgrade to the current designs rather than dump money into an old rig.