The graphics card is really pretty secondary as far as blender performance is concerned. Obviously you need enough to show whatever is in the viewport, but a mid-range gaming card like you have there will be just fine. Something to bear in mind is you wont need fancy shaders, just polygon pushing power. As such steer clear of the new low-range cards, which tend to cram in all the new features with less power, and instead go for a slightly older card that used to be high range. The ATI 4800 series, and the NVIDIA 8800/9800 series were all great cards for example and should give you a good bang for your buck, aim for one with 1024MB ram if you can.
For the processor, obviously it will make the biggest impact on render speed so get what you can there, also core numbers are very important as rendering (and more and more other tasks) is multithreaded - a CPU with more cores at a slightly slower speed is better for rendering than fewer cores at high speeds (well do the math, but generally it works out that more cores=better).
In my opinion though the biggest boost to what you can actually do with blender is down to RAM, it will determine the complexity of scenes you can load and render and reduce the amount of caching you will have to do to render large scenes, which also boosts render speed. If you can get 4GB, do (heck, get as much as you can really, when it comes to RAM you can’t have too much). Just remember you’ll need a 64 bit OS to utilise more than 2.5GB.
You asked whether the OS affects render times. There isn’t much difference in my opinion, I think in general Linux tends to be a smidgeon faster. Most likely this is not down to any inherent superiority, but just because linux systems tend to have less cruft going on in the background compared with all the superfluous 3rd party antivirus/firewall/malware/bloatware you typically find on windows systems. Ubuntu is a very friendly and easy to work with OS nowadays, and I highly recommend it for blender related stuff, if you aren’t a hardcore gamer I would say it makes a great choice.
Hope that helps. As for your selection, I’d say it looks pretty good. If you can afford to, up the ram to 4Gb, as that will really be a huge help though.
Edit: Oh and remember, the biggest effect on how good your renders will look comes from that component between the chair and the keyboard. A shiny PC helps, but until the devs finish work on that “make beautuiful render” button, it’s all about the artist really, not the tools.