Edbert - For gaming FX53s are THE fastest thing out there. They are socket 939 which is replacing socket 754.
Socket 940 are the 'old' (lol) FX51 and Opteron series of CPUs.
FX53s are expensive because they are still relatively new are are AMD flagship non server CPU. They are still cheaper than a slower (unless your encoding) top of the range Intel P4EE (emergency edition).
One tip for the future - Next year AMD will realase their 'dual core' cpus. This will be two cpus on one die. Thus eliminating any specialised software to really take adavantage of a more than 1 cpu setup. Essentially they are 2 cpus built into one. They will only be available for socket 939/940.
DDR400 is all you need, unless you are planning to increase the HT bus (no fsb). The main conscern is that is should be the fastest, currently CAS 2, memory available. You will also need 2 sticks to take advantage of the dual channel memory option.