Indeed. Is there a reason you are looking for an upgrade? You current card is great for even 1680x1050 which is over twice as high as your current resolution. You will see no benefit from a better card with your current monitor.
That said(despite your comment) it certainly would be a good idea to upgrade your monitor. Undoubtedly your current monitor is very small by today's standards and a switch to a 1920x1080 monitor will be an amazing difference for gaming both in screen size and detail. A good one can be had for about $200. For reference you current screen displays 786,432 pixels while a 1920x1080 monitor shows 2,073,600. The difference in detail can't really be overstated.
You current card would even still be ok for that resolution but upgrading to an HD5850 at that point is a worthwhile idea. Just get the 1gb version. 2gb is only really useful for even higher resolution monitors/eyefinity. The card can also overclock a huge amount, up to 1000mhz which will give you performance significantly higher than a stock HD5870. This would be a good choice;http://www.newegg.com/Product/Product.aspx?Item=N82E16814121375