For your price range and relatively high monitor resolution, I recommend the GTX460 SE 1GB model. The SE is a cheaper version of the GTX460, but it comes with 1GB of memory to run higher resolutions. You can overclock it and get really good performance. 550 watts should be enough to run it well, unless you actually have a couple hamsters on a treadmill powering your PSU.
You can run a lot off a 550w unit but it has to be able to foot the load. I also have a GTX460 1GB and a student my self so I know how it feels to be broke. The SE is slower but still a good card and should use roughly 15-20w less on a per clock bases if the sample is good. Performance they are about as decent as it gets. They are roughly the same performance as a GTX280/285 while costing a lot less and don't need the use of a 8pin. Just two 6pin connectors and you are set. Driver wise you are better off staying with nvidia as the drivers are stable even when they are slower than ati. I have used a fair share of ati cards so I know how they are but the newer you go with them the worse the drivers are. As for the ram don't worry 4gb is enough for most users. As for every thing else you are good but the psu is worrisome.