fx-55 single vs opteron dual?

You won't see significant performance gain on most daily programs, except the ones that do take advantage of multi-cores. On the other hand, it may improve your system's responsiveness, as more than one programs can be executed at the same time.

I would make the switch, then OC it to 2.8Ghz to obtain better performance  .

It's a cheap, effective upgrade.

I moved from single core A64 to an overclocked Opteron and it is much more responsive in Windows, plus games are more and more using dual cores now.

Don't spend a lot though - otherwise it might be worth making the jump to an Intel platform.

Thanks for all the replies. Ethel, how much would you spend on a new 939 dual core cpu before making a switch to a new intel upgrade?

A good rule of thumb when moving to a dual core processor over what is already an arguably fast single core for gaming- you need to get a dual core that is just as fast Mhz wise as possible. The dual core will seem slower at most games right now if it is not clocked very close to what you had as a single core. In other words, if you drop in a dual core CPU clocked at 2.4Ghz, it is going to make your games seem like they took a small hit compared to your old 55FX at 2.8Ghz.
Games that can make use of a dual core will do better, and the rest of your system will be faster, smoother, but for raw game performance, take these words of advice!
If you go dual core, (and you should if you can find one reasonable) get something that will clock at least 2.8 as well, or you will feel like you did not get your moneys' worth.

Of course this only applies to CPUs of the same architecture.

I upgraded from an Athlon 3400 single core @ 2.4 gigs to an x2 4400 @ 2.2 gigs and it spanked the single core system completely. Not even close. I couldn't believe it actually ran 200 mhz slower. It really was quite dramatic. And I mean in games.

My E6600 at stock (2.4) tripled my BF2142 framerates over my 3700 (2.6), although architecture comes heavily into play here as 2142 is single-threaded.