With $1200, you'll have to choose either high-end processor or good graphics chip. Can't have both.
I suggest MSI 1029 or 1032 depending on your graphics chip...
they're not designed for gaming.
More robust support of OpenGL (CAD-specific) features, like hardware overlays and clip planes.
Memory manager optimized for complex object models instead of high res textures.
Hardware certification so...
Well, the 64bit CPUs won't offer any advantage over 32bit ones in the software you're using.
Unless you're primarily looking to save a few pounds and have 64bit compatability in the future (at least a few years to come), I would...