Parts Preferences: Prefer Intel motherboard for better future-proofing. No preference on Nvidia/ATI GFX cards.
SLI or Crossfire: Maybe
Your Monitor Resolution: 1920x1080 (not 100% sure on that, but I'd be happy with this res)
Additional Comments: I'd like to run every game out there without too much difficulty. I prefer steady frames over 'Ultra' GFX settings, but I'd like the machine to be able to run everything on high fairly well and be fairly future-proof, but cost-effective. I also don't need a crazy large monitor with a high resolution.
And Most Importantly, Why Are You Upgrading: I'm using 2 laptops; the one I use for gaming has an i3-330M and a Mobility Radeon 5650. It's served me well but I'd like something more up-to-date. I can't really run the Witcher 2 for example. Also, I'd like to finally build a custom machine.
Red Flag on memory, do not buy obsolete 4gb chips, buy 1x8gb or in case of ACAD system get 2x8GB.
Your rendering requires a good cpu, have you considered i7-3820 ? The more cores the better, i5 is low for that purpose.
Next is your graphics card and here everything is debatable, because professional ACAD does not use crappy consumer video cards and monitors. Not sure they even switched to dumbed down TV resolution monitors. 21.5" will force you to buy glasses in a few years, the vertical space on wide screens is much lower than the previous generation CRTs.
Look at the 24", highest Samsung before going TV size to 32".
Make sure monitor has DVI input, do not connect HDMI. For me ATI(AMD) always had the extra performance in every field over nVidia and the later never caught up to specs. despite higher pricing and hype.