I've personally had no problems with ACAD 09 and Vista.
As far as hardware, the resolution is largely a matter of personal preference. More important is the video card itself. You will need a decent one, I would say with an nVidia chipset. And more RAM is never a bad thing. Get as much as you can afford, but I would run with 4GB minimum. Hard drive speed is irrelavent. If he is only going to be working in things like AutoCAD, SolidWorks, Inventor, etc., then a decent dual-core processor should be fine, as the graphics card will be doing most of the work. If he is doing any kind of rendering, i.e. Maya or 3DS or something like that, then I would go with quad-core. A general rule of thumb, so to speak, that I like to follow is spend about the same amount on your CPU and graphics card. Having an imbalance either way kind of sucks.