For CSS and source games, majority of the half decent gaming cards would be capable of running at max out 1680x1050, but for that budget, I would have to agree reflux 5770 would be a very good choice.
For mmorpg on the other hand, they tends to be CPU dependent as much as graphic card (probably even more so).
Unfortunately, for gaming it does work liking playing 1080p videos which you can offload the load from CPU to graphic card instead using hardware acceleration. The data ALWAYS pass through CPU first, and then pass to the GPU for processing the data and rendering the graphic. Think of the CPU and the GPU as two gates in a pipe, and data being the water going through the pipe. CPU would be the first gate the data reach so if your CPU is slow, it will bottleneck the data and making the GPU not being able to use its capability to the max. Imagine the first gate in the pipe is only 70% opened, and even if the 2nd gate is opened to 100%, it is still only having same amount of water passing through it as when it pass through the first gate.