Couple quick things:
1) Most people dont understand the term "bottlenecking". That's when one component has more capacity for performance that is unused because a lower performance component is limiting it. Honestly, you need a slow dual core processor to significantly bottleneck most GPUs. Tom's did a great article on this about 1-1.5 years ago showing next to no bottlenecking on cards as fast as a 5870 with overclocked dual core processors compared to OC'ed quad core i7's (not sandy bridge but in gaming the last gen provides nearly identical performance). With your processor at 3.6GHz you'll be just fine!
2) If you're not going to be gaming on multiple monitors, having those other screens up will tax your performance a little but it shouldn't be dramatic.
3) The faster the card the better the performance. I'd recommend the 6870 over it's little brother since you're at 1920x1080 and are hardcore (hardcore parkour!). The brand really shouldn't matter too much; just look for a good deal. Most factory OC'ed cards are not a good deal on newegg or other site. Think I saw them available on newegg for $140-$150 after rebate. When you're shopping around also consider the GTX 560, Nvidia's competitor to the 6870. Either are a good choice. If you can stretch your budget a bit you could go for a 6950 1GB, which would be a nice bump in performance.
4) Your PSU is plenty for any of the cards I mentioned.
Cheers! Let us know what you decide!