If you have friends with Tech savvy best to buy the parts and have them build it for free (or in my case I charge 1 case of beer/hour).
Really hard to find prebuilt systems for around $1k that are any good. You can buy the components and build them yourself or have someone build it. It's cheapest and if you can get a knowledgeable friend or family member it might not cost a penny to build.
I can build my own for upwards of 70% less than buying a prebuilt.
Check Dell's website and look for the Alienware prebuilts or check Voodoocomputers.com. Both allow you to start with a base computer than customize depending on what you want to spend. Alienwares start at $1200 for desktops and can go up as high as the $6k mark.