The Opteron is the server level series (there are three types: 1xx, 2xx, 8xx).
AthlonFX is for the enthusiast market, it's simply an Opteron 1xx.
Athlon64 is the consumer CPU.
Differences:
Opteron: 1MB 2nd-level cache, dual channel memory controller on-die, supports dual-CPU configs (Opteron 2xx), supports up to 8 CPU's (Opteron 8xx);
AthlonFX: 1MB 2nd-level cache, dual channel memory controller on-die, no multiprocessor support (like an Opteron 1xx);
Athlon64: 512kB 2nd-level cache, single channel memory controller on-die, no multiprocessor support;
To use the 64bit functions, you'll need a 64bit OS (Linux, xBSD, WindowsXP 64, Windows 2k3 64) and 64bit drivers (CW doesn't offer 64bit drivers for Windows yet, but we intend to support 64bit Linux drivers - 64bit Windows is still beta, BTW).
Since Scope is DSP based, I don't expect any performance increase by using a 64bit system, but it could help the host software (Cubase, Sonar, etc), given that the software would be 64bit compatible.
The AMD64 CPU's offer some nice features only available in 64bit mode (16 registers, 64bit integer arithmetic), and some other features also available in 32bit mode (on-die memory controller, SSE2, and SSE3 anytime soon).
The impending socket switch (940->939) only affects AthlonFX and Athlon64, the Opteron will keep the 940, and getting an Opteron would be the best choice, anyway...
And something to get an idea how fast that thing is: I use a single Opteron 140 on Linux/ AMD64, and playing Neverwinter Nights, I didn't even notice I was lacking hardware 3D accelleration 'till I finished the game - all 3D calculations were performed by the CPU...
