With this comp, you qualify for the free beta of XP Pro 64 bit edition on MS's website. CAD should not have any problems with a 64 bit computer, but I'm not sure.
I just bought a computer very similar to this one for myself, haven't had a chance to test it yet, I'm waiting for my case to arrive.
Rosewill Value Series Black ATX Mid-Tower Super Case with 350W Power Supply, Model "R103A" -RETAIL
ASRock "775V88" VIA PT880 Chipset Motherboard for Intel LGA775 CPU -RETAIL
Intel LGA 775 Pentium 4 630 3.0GHz 800MHz FSB, 2MB L2 Cache Processor w/ Hyper Threading Technology - Retail
2 WINTEC AMPO 184-pin 512MB DDR PC-3200, Model 35145588-P - OEM
Jetway ATI RADEON 9600XT Video Card, 128MB DDR, 128-Bit, DVI/TV-Out, 8X AGP, Model "96XT-AD-128C" -RETAIL
Maxtor 40GB 7200RPM IDE Hard Drive, Model 6E040L0, OEM Drive only
Rosewill BLACK 52X32X52 CDRW Drive, Model RR-521, Retail
Final price for this? $634.34 + shipping
Note: This is without a monitor. I recommend going to a local store to get one, as shipping would be very expensive for one. Its also without a floppy drive.. they aren't used much anymore IMO.
The processor and MB on this are pretty much cutting edge stuff. The computer won't dissapoint you, I think.
This is probably overkill for doing CAD work.. However, you probably wont need to buy another one for awhile.