click this, run the app, http://www.yougamers.com/gameometer/10278/
personally I think your card is your weak point there, but you should be able to play it at 'good' I have a x3 435 4gb ram, 5770 card and it says I would get good results so with a card upgrade you should be laughing.
It says it can't handle medium/low settings at 1920 x 1200 and just about can at 1680 x 1050. So low medium settings at 1680 is along way from maxing it out. You may get medium settings at your low resolution but to max it out you need an overclocked 955 and a 6970 minimum.