the answer is out here:
http://www.anandtech.com/printarticle.aspx?i=2381
1600x1200 (1.920.000 pixels) is nearly the same resolution as 1680x1050 (1.764.000 pixels), thus the performance on these resolutions will be similar.
Ati Radeon 9800 Pro has nearly equal performace as Radeon X700 Pro - this will comfirm that:
http://www.formulapc.net/news+article.storyid+260.htm
So look at X700 Pro results in WoW 1600x1200. You can expect 20-30fps.
Also mind, that, generally, in lower resolutions (up to 1024x768) you can have slower graphics card and good CPU and you will get good results, but in higher resolutions (1280x??? and above) it all depends on graphics card, what the performance will be. So, considering you want to play on 1680x1050, you would benefit from upgrading your videocard, f.e. Gf8800 or Ati X1950Pro/XT.