A 7850 is a substantial upgrade and will rock 1440x900. It will perform well even if you upgrade to a 1080p monitor, though. Your processor should be fine for now, especially at 3.6. The nice thing about the 7850 is that it also provides a reasonable upgrade path, since a pair of 7850s in crossfire is a good combination when you get a new CPU, motherboard, and monitor.
I would not recommend an SSD as an upgrade right now. They're nice but not the "core" of your system--game performance, to me, is much more about framerates and visual detail than it is about occasional load times. (I guess some other people might disagree.)
Finally, note that there are actually a few 7870s available for just above that price cutoff after rebate. Might be overkill for your resolution, but it's a substantial upgrade (though you can make some of the performance back by OCing the 7850 yourself). You won't see that in D3, but you might in Skyrim or other new games.
Thx for the response and the tip, unfortunately Prices differ quite since I live in Europe and we dont have rebates. So the 7870 isnt rly a option, id sooner go for a lower card 6xxx if it isnt so much difference on my system, but as you say the 7850 is a nice investment
If you are looking for better frames i agree with motornueron - SSDs may offer faster boot up but not more frames and besides the current SATA III SSDs paired with your board would be less than ideal Even thought the HD 4850 is long in the tooth, at your resolution try toning down settings for the more CPU intensive games