There are several posts on similar topics in this forum, and they're searchable. I have read most and posted several. I tend to build up custom systems simply because of the premium that pre-built system vendors place on their products. Since most of those have websites that allow you to configure you're essentially doing a similar thing, and while I custom build I source it all from a reputable local supplier and have then build the basic system. If you read through here and frame your requirements then attempt to configure according to a vendor's website then they will hopefully offer compatible components, using that as your shopping list I would price around and go back to the big guys if there is no substantial saving in using your reputable local computer outlet. In my case there always is.
Another approach that suits many is to go for a laptop, there are many very capable options these days. I would look at Dell, MacBook Pro has been reccommended, or HP have some desktop laptops that tempt me with their big 20 inch displays configured correctly they may be a good one to go for.
The consensus seems to be that NX is probably better on Intel, as I think there is some sort of relationship involved, though I could be wrong. Colleagues have dabbled with AMD in the past with results that satisfied their needs. On the other hand most people seem to think their needs are the most extreme, (hence the number of SUV's in our cities), and I thought these guys needs were moderate. For one system if you're not going to experiment then go with the tried and true. I have also read the the quad core over the duo core is much of a muchness as far as NX is concerned.
Get a plenty of memory, 8mb if you can, or at least leave room for expansion in terms of how you buy the modules per number of slots on the motherboard. Try and get in quickly as the prices may be about to go up, (something to do with dust storms from China drifting over Korea I was told). I would think XP-64 has the nod over Vista for most of what I have read.
Get a FAST a hard disk if possible, and yes I don't bother with RAID. What I do is have two hard disk a smaller fast one with the O/S and current projects, and a big storage drive for the bulk of data used less frequently and my backups. I don't partition the small disk either avoiding any associated issues with system stability if anything does go wrong. Recently we posted about I think is was a 60Gb or 80Gb SATA runs at 7200 rpm, would offer the best bang for your buck short of SCSI which probably breaks the bank.
Get a decent graphics card the best you can afford. Look up the certified hardware guides from PLMS before you buy anything, but especially these. I have often fallen short of the budget required for some of the certified graphics cards, but any well rate Nvida GeForce, or FX series cards with 256Mb or 512Mb. of memory on board have worked flawlessly. Avoid ATI unless you can find a certified model or specific recommendation to counter the bad experiences of myself and others in the past. Ignore gaming statistics you'll find a lot of good benchmarks for all kinds of hardware on sites like tomshardware, but the graphics card benchmarking seems to relate mainly to gaming which has distinctly different requirements to CAD.
That's about it. Get yourself the best size and quality of monitor that you can. If anything from what I have read you're probably better with a bigger wider flat panel, than using two smaller screens as some people seem to like. There are several posts about problems with it so I tend to think who needs the hastle.
Best Regards
Hudson