I have a 6800NU in my second computer that will run wow at 1600x1200 at 40+ fps, i think your x800gto2 should be ok.
what type and speed of processor are you running, and how much ram do you have.
the 6800NU box runs with a 3200+ (@2.4ghz) and 2 gb of ram. wow is a cpu hog, and eats 700+ MB of ram when draw distance and settings are maxed. While in huge towns with lots of people and or huge raids in large dungeons wow lags no matter what. With a lot of people around wow has to keep track of every peice of armor, every spell effect, roll the proverbial(sp) dice for every action, track all movement aggro, every swing vs every armor rating buffs, debuffs, AI etc... CPU and RAM rule.
This is also one of the reasons I'm going with a x2 3800+ in my 7800GT box, my friends CPU usage went from 100% single core to 65% with the dual core upgrade. WoW may not be dual core optimized, but the OS could put other processes on the second core. Even on this machine I'll get drops in the 20's, thats just wow. The newer weather effects dont help either.
GPU comparison at 1600x1200
GPU benchmark
CPU comparison at 1024 x768
CPU benchmark
the test computers all had 1G of ram
Looking at your rig i dont think its you CPU, whats you peak commit charge after playing wow in you task manager performance tab. If you running other stuff you may be pushing that ram.
mine will go above 1.1GB if im browsing on my second monitor (2 or 3 windows) vent running, and you typical background apps