At those resolutions, no need to upgrade! I see no reason why it should be a problem, unless your CPU turns out to be the bottleneck. Yours is 1600x900 so I really doubt you'll have any issues. You won't manage to use more memory than 1 gb as that's the limit on your GPU. For that, like HEXiT said, if you run out of memory or you experience some lag, just turn the draw distance down a bit and see.
For more info, have a look at this:http://www.tomshardware.com/forum/113964-13-requirements-info
All the best!!