From what I gather the 32 bit v. the 64 bit starts with the operating system. As the articles say you will need a 64 bit processor, such as the athlon 64 to take full advantage of the architecture of the 64 bit operating system. Someone correct me if I'm wrong but a 32 bit processor will still work with a 64 bit operating system.
While technically true, I wouldn't want to try to run 64-bit Windows on a 32-bit system. It would be a performance nightmare. At this point in time, I wouldn't even buy 32-bit hardware - it's obsolescent.
And DO NOT go with 64-bit Windows if Autocad is important to you (at least as of right now). Everyone I've heard who's tried that has had problems. That should change in the next year or two, though, with Vista about to come out. Given Autodesk's track record, though, I would be very leary of jumping to Vista until you know someone else who has successfully managed it...
And if money is tight, GO WITH CPU, not a RAID. The best CPU you can get is the most important thing. Still, try to get a fast hard drive. SCSI tends to be fastest, but also most-expensive. Keep in mind that a single WD Raptor will work great, and is not too much.
There's also the new vertical-write drives, but I haven't heard much about them yet, so I don't really know the full scoop on them... Since they cram more bits into the same amount of space, though, they are supposed to get performance similar to a 10K Raptor, even though they only operate at 72000RPM