Would you build a machine with less than 4GB of RAM?
Would you want to be limited to a 2TB HDD?
Would you want to miss DX10/11 feature thereby loosing alot of your graphics card potential?
I think Windows 7 is the better choice, you get the most out of your hardware, isn't that what a gamer wants?
As you have asked me directly, here are the answers:
4GB of Ram is more than I currently have on my Netbook, which has 2GB of RAM. I'm not sure what program would require more than this, I don't currently run any that do. That said, I'm unfamiliar with the requirements for modern games, so they might have scaled to the point where anything less than 4GB is insufficient. I believe that about 8GB is not uncommon these days, but I don't know what it is being used for. Nothing that I have today needs anything like that much.
2TB of hard drive space is massive. I use a 40GB SSD (Mtron Mobi, very snappy hard drive) in my netbook and that limits me as things stand now with how many games I install.. and this is for older games. So less than 2TB would be fine be me, and anyway I'd pick SSD for quick access, which does limit storage space anyhow.
Not having DX10 or 11 could be killer though. This alone would push me towards Windows 7 and from what others have said here, it looks like the best choice for me.