I would go with the Opteron, it's specifically built for servers and workstations. I'm not familiar with the 800 series, a quick check on newegg listed the 885 at $2,200 so I'm assuming it's a high end server processor, either way at that price I would think you would want to stick with the 200 series.
The FX series is mainly designed for gaming, and in my opinion they're not worth the price. The FX-55 was $1000 a year ago and now you can pick one up for $200, it really doesn't seem worth the price to me considering how quickly they become obsolete.
I would say almost any generic CPUs will do fine wouldn't it? Unless you have specific needs. E.g. if you do lots of floating point calculations, you'll have to look at the processor architectures (or ask someone who remembers them off the top of their heads and have no life ) and find one which has more FPUs If your needs are really specific and you need vector calculations, you might even be able to adapt a graphics card for your needs.
But for a general purpose computer, I'd go with Core 2 Duo, seeing that it is the current performance leader
Ok, so the FX is essentially a crippled Opteron. The opteron has 3 Hypertransports, so does the FX, but the FX has two of them disabled. The opteron 200 series is a max 2 way proc, and the 800 series is a max 8 way proc. I would go with a dual core opteron.
Dustin has touched on it a little above, but pretty much all the current gen offerings from intel and AMD are the same, the difference mostly between ranges is how many processors you put on a motherboard as well as certain features disabled on low end chips. There is also the issue of what sort of memory do you need. Of course really you need to look at the applications that will be used before deciding. I expect in the end you will want to be looking at either a dual processor, dual core Opteron 200 or 2000 series or Intel Woodcrest setup with ECC, registered or fully buffered memory.