~$1300 Gamer Build

I'm going to correct somethings to start with. Dual 5850s don't need 750W to run. ATI recommends a 600W unit with 4 PCIe connectors. The problem is that most 650W units only come with 2 PCI connectors, making them sufficiently powerful to run dual 5850s, but you'll need to buy some adapters (though 5850s might come with some). That's why it's generally recommended that you get the 750W units, since they will almost always have the required 4 connectors.

@Valoric: The 5870 doesn't cost $600. It costs $390 (outside of combos). Dual 5770s will run $300, but use up an upgrade path, use more power and generate more heat. It's advisable to go with a single card to start a build with. Dual 5570s will defintiely run on a 650W unit.

On to the build. Here's a couple of recommendations. First, I'd drop the i5 down to the X4 955 and a Gigabyte GA-790XTA-UD4. That'll generally save $100 and cost you nothing in terms of performance. Also, you preserve an upgrade path for the CPU, as the LGA1156 socket is going away in the next year, while the AM3 socket is sticking around for a couple of more years. That would either allow you to either upgrade to the 5870 (which you won't need) or just save some money.

Since your getting a 1080p monitor, I'd definitely stick with the 5850. It'll be powerful enough to play almost everything at that resolution, so you won't have to worry about needing more power if you get into other games.

I'd also consider switching the case/PSU to either the Antec 902 and Earthwattts 650W ($145 after rebate) or the Antec 902 and TruePower 750W ($190 after rebate). The 300 Illusion is a great case, but it's a little small. It won't really affect the components, but I always like to have a little extra room.