Well, in that case you should just go for a GTX 580, I only know the UK prices, but I'd be very surprised if it wasn't in your price range. If you prefer AMD you could go for a 6970; my personal preference is the 580, but on average they are quite level, superseding one another in different games. If you are interested in 3D Nvidia is definitely the way to go; furthermore you have PhysX with Nvidia, sadly it's not used as much as I would like, but it still makes a big difference to games that support it.
If you are looking for multiple monitor setups AMD is the way to go.
Looking at your general specs, your Corsair 750W PSU can be taken down to something like a 650TX without causing any issues. I'd also get a slightly better motherboard (money saved in one place, invested in another!). Furthermore, for the sandybridge platform, you only need a bare minimal CPU cooler. Unless you're heavily focussed on OC'ing (4.7Ghz+ and such), a cheaper heatsink such as the CM Hyper 212+ will work fine. I'm not sure what ambient temperatures you experience over the summer, but when I was testing the 212+ with sandybridge I found that an ambient of 35°C with a 4.5Ghz OC on Prime 95 and the CPU started to run at 70°C.
You could also invest in an SSD if you're interested, that could provide a viable improvement rather than a motherboard upgrade, that is if you're interested in changing any aspects of your build
.