Probably the best bang for the buck is an A64 3700+ or 4000+ with unregistered ECC RAM. You'll save tons off the opterons, and it's the same basic processor. You only lose out on registered memory.
dualies are for servers that are hosting multiple small games like for companies that sell out clan server space and they really have multiple like 16 player game servers running on a single machine. In that case, two CPUs actually work to balance the load. But if you're trying to host a single 64 player server you really only need ONE good CPU. The game server app is single threaded just like the game client app, so dual CPUs only buys you some offload of the networking and miscellaneous overhead type work, and not a whole lot else.
You can minimize networking overhead by using a good server oriented nework card (i.e. not the $15 gigabit special). Intel server cards are pretty highly respected.
We haven't seen the game yet so it's hard to say for SURE that the AMD architecture is going to outperform the Intel architecture, but it's a safe bet based on previous AMD/Intel performance in games.
If you were to go with a dually setup, I would think the x2 4400+ would be sufficient. Certainly no need for 4 cores like the dual 265 setup 'theman' suggested, in that case two cores will be doing diddly because BF2 will take up one whole CPU, the networking and miscellaneous activity will fill up a fraction of the second and the others will be twiddling their thumbs.
With the introduction of x2, I really don't see a need for Opterons aside from business use. There is nothing special about their architecture that makes them better than A64s, and I think consumer level hardware has reasonable enough reliability for this kind of application.