Am a Mechanical/Piping Engineer who is into Engineering design that requires graphic softwares.I really dont know much about the various computer processors(AMD,Pentium,Centrino,intel core etc).Pls educate me on the best processor with regards to the above.
You get a lot of bang for your buck in the CPU department, but what you probably should be looking at for any CAD work is a OpenGl graphics card. These are workstation graphics cards and not gaming cards. Here is a gander at Newegg. These cards are a lot different from gaming cards and so don't be disappointed if you buy an OpenGL card and your games are not top notch, but the work you do will blow you away.
Find out if the software you are using is multi threaded, if it is you will benefit from multicore processors.
The Athlon II X4 620 has 4 cores and it's less than $100 dollars.
What Yoder says about the OpenGL card being best for CAD is true, but depending on what you're doing you could get by with a good gaming card with a lot less money of what you would spend for an OpenGL video card with similar specifications.
I agree with dunklegend and Yoder54. When it comes to any type of graphic design, AMD will always beat Intel, and as dunklegend says it is a lot less expensive than Intel.
One thing to take in consideration is:
What software will you be using?
Will there be a lot of high quality rendering needed?
If it is to be medium sized average design then an Athlon, although I recommend something a little more updated like the Athlon Phenom X4 which is around $100.00 also, with 4GB of RAM, although the 32 bit OS will not allow you to allocate more than 2GB of RAM to your design program. And as far as a Graphics Card, an OpenGL is a great option, but as dunklegend said, they can get pretty pricey (from a couple hundred to over $1000.00). I personally recommend ATI or NVIDIA. A good ATI Graphics Card is the ATI Radeon HD 4770 and it is going for about $100.00.