I think it probably is that guild wars isn't fully optimized. My card seems to be running great outside guild wars - the average 3dmark score with a 660 ti is 8560. I have a pre overclocked one so it makes sense that mine is over 9k.
I have been running all guild wars options on the highest possible choice - "best looking" with super sampling. I kept an eye on gpu and cpu usage when I was getting the poor 20 fps: cpu load of both cores was about 70% and gpu load was 40%.
Well it looks like your card is fine since you have a higher than average 3dmark 11 score. 3d mark 11 stress your GPU much more than Guild wars 2, so I don't think it is a power supply not providing enough power either. The fps you are reporting in Guild wars 2 seems ok since you are playing everything at max settings. Again, keep in mind that I'm using a SB-E i7 3820 @4.3 ghz and my dual GTX 670 SLI (boost clock 1277 Mhz/6.4 ghz memory) only gives me 50-70 fps most of the time at 1920-1200 res max settings on everything, so your single GTX 660 Ti giving 30 fps in some area is about right. This game is still very new and not optimized yet. Future driver updates will probably give it major boost in performance.
Another factor that is also impacting your low fps in Guild wars 2 is the dual core E8600 CPU, you need to overclock that CPU to at least 4.0 ghz or higher because Guild Wars 2 does utilize Quad Core CPUs and higher frequency will yield better fps. Upgrade your CPU to i5 3570k is probably your next option to fully enjoy that 660 Ti.