This might be a bit disjointed but it does all go together but not so much in any order. I buy parts rather then systems but it may help and I enjoy blabbing about it. as you will see...
Once there was a noticable difference in the performance between the AMD and Intel chips. Now though both are running fast enough cheap enough that to base a decision on brand is really more brand loyalty. Simply buy the fastest that your budget will allow. There is a price bubble where an extra 20 dollars will get you the next chip up but the one up from that is an extra 150 dollars. for myself and my budget I spend the 20. depending on where you shop and such it is generally around the 3.2 gig mark, which is really alot of raw speed.
What about Xeon's or Extreme editions or dual cores? depends on what you want the machine to do. Xeon/Opteron are meant for servers, these have better memory management built in and other tweaks that are geared to file servers. The Extreme/FX chips are power number crunchers, they would make darn good work stations but have a premium price and unless your rendering and doing heavy analysis or have extra cash on hand,probably not worth the price. Last the dual cores. these are meant for multi tasking multiple applications, do not expect them to run single application any better then the single core chips (not yet anyway) unless you are running an operating system built from the core up to utilize multiple processors (unix and the like). soon applications will be written to take advanage of both cores but Autocad is not one of them yet. For the budget you mention I would stay with the P4 or Athalon 64 chips as fast as I can afford.
Video is a weakness of mine, I have to have the best I can get my hands on (I enjoy the games, Half life, Doom, quake etc...). Games and rendering are the only reason to go top of the line though. Both Nvidia and ATI have released not one but two major video chip models in the last year. I have a BFG 6800 OC deluxe on my machine at home at there is nothing it can't do, it is before the two releases that I mentioned with those about triple the performance that I am currently running. The point to this is that you can get more video then you need at a decent price ($200 to $275 compared to $500 and up). Getting a card without a DVI out is getting hard but I would make sure that it has two (for a second monitor, vga to a flat panel just doesn't quite make it). For cad i perfer ATI based cards, There is something with Nvidia cards and mtext that make the text hard to read when you edit it. For a pure gaming machine BFG tech makes the finest(These are Nvidia based). For ATI a 700X or 800X and Nvidia a GeForce 6800 would do the trick
Memory is fickle, buy it now and buy enough. as your experience has shown it may not be easy or cheap to add later. The manufacturing technologly changes fast and it is only going to get worse for a spell. Its another discussion altogether as to why.
I would not worry about dual core for these machines but probably the ones after these, if you can afford the Exteme or FX chips it will be longer before you feel the need to upgrade but not a deal breaker for what you are after and the extra money might be better spent on more memory (2 gig mimium). The Xeon/Opteron just is not the right choice for anything but servers.
This is my opinion and I know folks have different ones. That monitor size you stated is a killer though, Viewsonic make a 20" around $500,any larger and and you are going to have to sacrifce some computer. There are large flat panels a decent price but be sure that is has dvi inputs or you will be disappointed in the image quality.
Hope it helps... Sorry I got long winded.
