Advice on Drafting/Gaming computer?

Hello, I am currently thinking of getting a new computer, but I am not entirely sure what I want. I will be using it frequently for AutoCAD 2011 drafting (2D as well as 3D) as well as gaming, so I would like something that has a decent amount of performance, ideally around the $400-$600 range (I can go outside of this range, but this is generally about what I'm hoping to spend). In terms of gaming, I don't need something that can run every game ever made at maximum settings, but I would like a computer that can run the majority of modern games at least at medium or low settings with no issues.

I'm not really sure what kind of computer I should be looking at though, or what kind of specifications I want. I'm totally clueless when it comes to computer specs and hardware...I actually bought my current computer ages ago thinking it was top of the line and it turned out to be a hunk of garbage haha, so this time around I'd like some more knowledgeable opinions! I'm mostly looking at pre-built things just because I have absolutely no clue about all the customization stuff, so I'd prefer to take the easy route lol. The most confusing part for me is video cards, I have absolutely no clue what the difference between them is or how to tell if one is good or not. Does anyone have any suggestions of either minimum specifications I should be looking at, or any links to computers that might work for me? Thanks a bunch in advance!

*edit* I just saw this form to fill out, so I put it below

Approximate Purchase Date: Pending

Budget Range: $400-600

System Usage from Most to Least Important: Drafting with AutoCAD, Gaming, General Internet/Word Processing

Parts Not Required: Keyboard, Monitor, Mouse, Speakers
Preferred Website(s) for Parts: newegg.com

Country: U.S.

Parts Preferences: none

Overclocking: I don't know what this means lol

SLI or Crossfire: I don't know what this means either

Monitor Resolution: 1440x900