That motherboard and processor look very good. (Check out Tech Report's articles on them:
http://techreport.com/articles.x/20188 &
http://techreport.com/articles.x/20911.) This Core i5 processor has turbo mode which will be handy for CAD applications that can only effectively use one core (it effectively overclocks some cores, and underclocks other parts of the chip, in order to maximize performance while staying within thermal limits).
The motherboard has 4 video outputs (3 of which can be used as DVI, with appropriate adapters), so it's likely that it can run dual screens from the on-CPU graphics. It also has a graphics-switching mode (like laptops), so you might be able to pipe the output of the graphics card through the motherboard, on the fly. (I've never seen that on a desktop, so I have no idea how well it works.) You could definitely try it only using the integrated Intel graphics first—the Core processors with on-CPU graphics are quite impressive, and actually stand a reasonable chance of working all on their own.
The video card is surprisingly inexpensive, and at that price point, you can afford to upgrade in a couple years if it ceases to meet your needs. (It's got driver support for CAD applications, which is nice.)
The RAM, optical drive and power supply look fine; no sense going crazy with those.
What's the total price of the system, and will the existing hard drive be the only mass storage? Any plans for using the HTPC as a backup server for the desktop, or vice versa?