You can get Windows 7 for $99.00 at Newegg: Newegg.com - windows 7 oem
. However, it is OEM software, and it links the software to the motherboard. So, if you decided to change out the Motherboard you would have to call Microsoft to release the software from the MB. That would save you $150 or so than buying the full version.
Also, monitors are not that expensive ($130 to $250), unless you want to go with 3D, then you are looking at $350 monitor at the minimum if you want a quality monitor. There is one for $200, but the reviews are showing that it is crap.
As for processors, I would recommend the Intel Sandy Bridge (i5 2500k or i7 2600k), OR I would wait until September 9th or 19th, 2011 to see what AMD bulldozer offers in comparison to Sandy Bridge performance wise.
Note: I would recommend a water cool solution for your processor. All you have to do is put it on your processor, and you are done. No hassle with the normal water cooling solutions: http://www.antec.com/Believe_it/prod...d=Mjc2OCYxNw==
Video Cards, is a crapshoot. Either AMD or nVidia would be fine with the current generation of vid cards. They both perform relatively the same with gaming. If I had to choose right now, I would go for AMD HD 6970 or GTX 570 (GTX580 if you don't mind spending $150 more).
Note: nVidia has a better 3D standard than AMD has adopted at the moment.
As for a case, I have always used Antec cases. I currently use an Antec 1200, but will soon get the Lanboy Air. I always opt for full tower cases, due to Vid card length issues presented with other cases. Personal opinion though.
With the configuration I would have in mind for you, you would be looking around $1500. It would last for years of gaming. Let me know if you want me to get a build built..
Note: I would recommend waiting for AMD's Bulldozer processor, and the next gen of Vid cards to come out. Next gen cards will run 2.6 times better than current gen. And, the bulldozer processor could provide a huge leap in performance. Plus, it is only a month away from release!