The Opteron is primarily a server processor in that is designed for 1-way, 2-way 4-way and AFAIK 8-way configurations (but there is no reason why you could not use it for other purposes).
The main problem now, is that the socket that the opteron uses (Socket 940) is now no longer used in the mainstream (i.e. desktop market) and so the motherboards will cost quite a bit, and the technology may lag behind the desktop market and/or certain features e.g. SLi graphics will probably not be available on these boards.
With regards to the Athlon 64 platform, if you are considering an Athlon 64, the best choice is to wait until after christmas and get a Socket 939 PCI Express Board