I've been really happy with all of my purchases from
cyberpowerpc.com. My wife and I both got new computers from them about 3 years ago, and had them both shipped to us, all for less than $1K. Not bad at all for 2 PC's, especially considering what we got. I was so pleased I actually ordered another one from them about a year ago for my mom. She loves hers, too.
Honestly, take a look at their system configurations. Play around with them and build one that you think you'd like. You might be surprised at the cost. When I got my PC's, I could not have bought the individual parts and put them together myself for as cheap as these guys had them. And it's already built, and tested to make sure everything works. Sure, you can get the parts yourself from somewhere like newegg, and it may even be cheaper. But if you've been out of the loop for a while, or you aren't sure about whether all the parts are compatible or not, then its nice to have a site like this, where you can be sure everything works together.
And if you get lucky, you can usually find one that has free shipping. Which is a huge deal because shipping a full PC is expensive.
As far as OS, you might as well get Windows 7. Most places like cyberpower are including it anyway. And it really is a very good OS. I'm loving it so far.
Just play around with those configuration tools, and post a couple of links to builds that you like, and someone can give you some feedback on what you might want to change or consider.