depends what CAD you’re wanting to do really. If it’s just 2D AutoCad stuf, then you don’t need a great deal. If you’re going 3D (say, solidworks) and needing to do reasonable size assemblies (prob 10+ parts) then you should really be looking at a MIN of 4Gb RAM, 64 bit OS, and a specific graphics card, ideally running to the OpenGL standard natively in hardware, though that epends to an extent again on the specific sware pacakge youll be using as Autodesk products I think can natively do DirectX. Either way, you need the seperate card really, ideally a minimum of 256Mb RAM, ideally 0.5Gb. Oh, and a high screen resolution of at LEAST 1600×1050
Search:-
x-stock
http://stores.ebay.co.uk/X-stock-shop
e computers
http://stores.ebay.co.uk/eComputers-LTD
both the above very good rep. & you can email them with specific requirements and they’ll see what they can get.