But I don't know what to get. I've looked around, had a lot of different ones suggested to me, but I just have absolutely no idea where to begin.

My needs:
Can play games like TF2/L4D2 (hopefully on highish settings)
Preferably has a 500GB 7200RPM HDD
4GB of RAM
Intel Core i7 (the newer series)
15"/17" monitor. Either works. Though I would like a 17" more

My price range is no higher than $850, so I'm hoping I can get what I want for under or around $850 :/

For those asking about brands, I don't have any specific preferences, but I really like ASUS :X