When doing design work any higher-clocked dual-core or quad-core processor would be your ideal processor. Most software still benefits more from the faster clock speed than multiple cores. Still, if you are planning for this computer to be in use for the next four years or so, I would still suggest going with the quad-core, as software is beginning to transition to take advantage of multiple cores better.
4 GB of DDR2 or DDR3 ram minimum, though this would require a 64-bit operating system to recognize and utilize all 4 GB remember. But you also don't always need to go overkill with the RAM.
This article by Tom's Hardware explains that most all computer applications (even though this is focused towards benchmarking and gaming) still runs fine with just 4 GB of total memory, so having more may not mean having more power.
I have always preferred ASUS brand motherboards, though in recent years it seems some quality issues have begun to lack. Many many others are starting to use Gigabyte-brand motherboards as they are just as good quality (if not superior in some cases) and less cost. I've used both and like them both. If you are doing something with design work, find a motherboard that offers at least one PCI-Express x16 2.0 slot for a good video card.
And for video card, I recommend nVidia Quadro FX line of cards, specifically the FX 570. It is a very decent card for design work, is specifically designed for more industrial design tasks than gaming, and has good driver support. I've built four systems using this video card and have had no problems with them. They are a little more expensive than similar nVidia GeForce cards, but that is up to your choice.
And finally, if you are building this computer yourself, and foresee perhaps getting in there and working on it occasionally at least, then don't skimp on the case. Find something at least decent quality. There are several good options out there and it all depends upon the space that you need the computer in and the components you are putting into it.
