Op,
I have played with building a Hackintosh. I own an older mac pro and built an equivalent machine it was somewhat cheaper and a lot slower. The reality of the situation is if you need speed and stability you need to use apple certified components and at that point you might as well buy one. The comparison between an imac and mac pro is not fair it is not in the same ball park at all. You can get a base macpro for 3000$ and add a lower end monitor and plan on upgrading later. I don't know what type of work you do but my pro quad xeon 10gb ram still is faster than most off the shelf PC even after 3 years. I often have video playing email photoshop with 40-80 36mb images open FTP software and web browser open all at once with not much drag on anything.