I had a small company building gaming machines in college and still do the same as a hobby today- I can say for budget you can build under $800 and get a killer machine ready to handle just about anything you throw at it.
You can PM me if you want to take the gritty details but a few things based on all my builds:
Motherboard- don't skip on this, its probably the most important part of your build, I prefer the brand ASUS, but plenty of competitors offer solid boards
And about RAM, personally gaming, watching movies at the same time/streaming 8GB should be sufficient, I see about a 40% load when I do this (battlefield 3, Guild Wars 2 for example). The 2011 socket will support up to 64GB and its really cheap if you hit the ceiling over time.
And Tom's hardware is pretty good, but always remember you reach a point where the cost will not yield a big benefit until years down the road when that piece of hardware is near worthless and something new has come along, so I would try to find the sweet spot with each component. CPU under $300 range USUALLY yields a good chip, and GPU, try to keep around the $200 mark and you should be just fine for the next 6-12 months until something kicks its butt, but again completely depends on how picky you are with the visuals.
Ok that was a pile of info, but again, feel free to shoot me a PM.