For gaming an AMD Athlon64 is definitely what to get (but you apparently know that). For today's games, the FX55 is the fastest available. The FX53 (and it's non-FX 'twin' the 4000+) are a little slower (200mhz) but still darn good for games. Any A64 you get will be good for games. If you don't overclock, then get a faster chip (3500 or 4000), if you overclock, get a slower one (3000 or 3200) and overclock it to the faster speed.
Dual core... (4200+ to 4800+) Today's games can't take advantage of them (some exceptions, but they don't show much if any performance improvement that I'm aware of), but in the future they will. It may take a couple years, but I think the first games to benefit from dual cores will be out in about a year, and it'll take another year for the technology to mature, so I'd guess in 2 years, a dual-core will be quite useful (if not a necessity).
Does that mean get one now? I don't think so unless you plan to keep your system unchanged for more than 2-3 years. They do have other advantages today - you can rip a dvd while you game away for example
, so that decision I have to leave up to you.
Remember, you can get a socket 939 board and a 3200+ today (that's a pretty good gaming machine...), and in a year, get that 4800+ and just drop it in... Might be a good idea pricewise.
In any case, once you get a fast-ish cpu (any of the above), your biggest bottleneck for games is the graphics card, so if money is an issue, get a slower CPU and a faster GPU.
Mike.