Ok, I think I have a succinct answer for you. If you go to this chart
, it explains the clock speeds and the L2 chache sizes of the various Athlon 64 processors.
Now in your case, you currently have the older Newcastle core (130 nm manufacturing process) with 512KB of L2 cache running at 2.2 GHz.
The X2 4600+ has 2 cores with 512KB each and each of them is clocked at 2.4 GHz (wouldn't offer a noticeable upgrade to what you have for current games).
The FX 55 has a full 1 MB of L2 cache and it is clocked at 2.6 GHz. The newer San Diego core also has fairly significant improvements over the older Clawhammer core of the first wave of FX 55's - they are more energy efficient too (check the wattage ratings in the chart).
If you are buying a processor for the games that are out now and the ones that will be coming out for the next year, I would say stick with what you have, or if you can afford it and want top of the line performance, go for the FX.
If, instead, you want to get a processor that will support the multi-threaded games of the future (I don't expect a big wave of those for at least another year, you should definitely go with the X2...but again, I would probably wait until a multi-threaded game comes out that you want before buying an X2 since the extra core won't be put into use on current games and the same dollars you want to spend today will get you a much better processor down the road.
I'm definitely waiting until more games come out that support multi-threading before going down that road...and I will also make sure to buy a processor that has 1MB of cache for each core! (It would be like running two FX's.)
I hope this helps your decision further.