The HD 5770 / HD 6770 will work with a 400w PSU, Your Xigmatek is not bad.
The HD 6670 should suffice for your resolution, but you should look at the benchmark if you have not done so already to get an idea.
Buying the HD 5770 / HD 6770 will last you longer since they are more powerful especially when/if you decide to upgrade to a higher resolution monitor. Assuming you stick with a 1240x1024 monitor, the HD 6670 will probably last you about 1 year for with maximum settings before you need to tweak it down a bit. The HD 5770 will probably be around 2 - 2.5 years before you need to tweak down the graphics a little bit.
Of course if games all new games are going to be as demanding as Metro 2033 or Crysis 2 with the DX11 patch and the high resolution texture pack, then everyone is going to need to upgrade a lot sooner than they thought.