i highly recommend an antec power supply at least a 500 watt, on the cpu it just depends on much you want to spend, i don't know much about the opteron dual core, but you might check into an athlon 64 x2 dual core
The X2 3800 is a good proc and i would go with that one. power supply wise Enermax. The Enermax Liberty 620W is a great PSU. It's modular and is just an all around awesome PSU.
While Opteron procs are nice they arn't ment as normal desktop procs. That and the specs are so close to the X2 and FX chips is not worth buying a server proc.
Also Antec PSU are nice and every thing but I have done some work with the A8N32 and have found that the Enermax PSU's work best. there are other brands but Antec isn't one i would suggest for this particular mobo.