Looks like you've done a lot of your homework, you did good on the parts picked. What kind of work are you talking about? Anything that would need to use some more of the budget or is it all left for the GPU? If it is, the natural answer is get the most GPU for the money. You have enough left to snag a 5770 which will do pretty good at 1920x1080. Some heavier games might not max out on that, but the next step up is $300. So this is where I'd start if you can burn the rest of your budget: http://www.newegg.com/Product/ProductList.aspx?Submit=E...
The 5670 will be vastly underpowered if trying to game at 1920x1080 with any kind of settings. If all you want is low details and shadows, yeah, probably can muscle through it a little bit.
I also looked at the 4890s and GTX275s, but looks like inventories are shot. So its the 5770 or a little less capable 5750 or wait for the Nvidia cards to come out and see what they offer or how they effect prices.
There's also 4870s 1gb left that do a little better than the 5770, but the 4870 isn't DX11. I think there will be value in that down the line.