GTX 760 is the best value for money right now. GTX 770 is faster but also much more expensive.
GTX 760 would have the added benefit of giving you awesome SLI performance at a reasonable price if you can afford to buy another one later. However seeing as you have just one PCI-e slot on your motherboard, SLI is not an option. That would make the GTX 770 a bit more attractive as a single-card solution, but it's way above your budget. It is, however, the same price as the HD 7970 recommended above, but faster and quieter.
If $400 is not too much for you, get the GTX 770. If it is too much, get the GTX 760 or GTX 670 (if available). You wont be disappointed.
And please don't forget to close the thread.