What CAD program are you using and what exactly are you doing with it? I took some drafting classes and we used AutoCAD (2004, I think?) being run on crappy old 1.5 GHz P4 Willamettes with 256 MB RAM and a 32 MB GeForce MX440 and it ran just fine. Of course, we were not doing 3D or anything.
If you are budget-constrained and want a decent unit now, here's what I'd suggest:
CPU: Athlon 64 X2 6000+ 89W
-not the 125 watt unit.
Motherboard: ATX motherboard with an AMD 690G chipset- just make sure it has a PCIe x16 slot (most do.) Alternatively, a NVIDIA 570 Ultra board will work too, but the AMD chipset boards are less expensive and run a cooler.
RAM: 2x2 GB DDR2-800
GPU: You can use something like the ATi 2900 Pro with 1 GB VRAM and it would do okay, but if this is a real CAD workstation, pony up for an ATi FireGL or NVIDIA Quadro card as these work better with OpenGL.
Case: Get a decent mid-tower ATX case.
HDD: Up to you, but DON'T use RAID 0. A 320-400 GB SATA unit isn't very expensive and should serve well.
PSU: Depends on the GPU you pick. You seem to want a higher-end one, so 600 watts should be sufficient. Just make sure to get a good one, not a $65 special.
OS: I'd highly
suggest 64-bit Linux if your CAD apps run on it, otherwise, Windows XP 64-bit. You'll need a 64-bit OS to use all 4 GB RAM.