For best results in rendering, You should have as many cores as possible, Best to have any Quad-core with sufficient amount of RAM say at least 2GB DDR3, with some professional graphics cards like Quadro/FirePro series.
I have intel corei7 920 cpu with 6GB of ram,and a 2*2GB DDR5 vapourX VGA but still I am not getting the speed that I want. Should I buy a server with dual AMD opteron 12cores or 8cores or Xeon would b a better server?
Which speed are you referring to.? If you are referring to final rendering, then it totally depends on the CPU.. If you are referring to desktop rendering then it depends on the video card.. It looks like you are on a crossfire setup which is no way beneficial even with the 4 GB frame buffer that it offers.. If this build is purely for professional use than i'd recommend you to use a single video card.. For further suggestions and advices, it'd help if you could mention the software you are using..
V-Ray is a tasking computation.. You require a good enough CPU to handle it smoothly.. 3DS Max has the ability to crunch up cores.. Until we know your budget, recommendation would be to pair up an i7 930 with 12 GB of DDR3 RAM and an ATI FirePro V4800 GPU.. Below are the links -