What frame rates are you getting while fighting normally? Cuz thats really the frame rate that matters to your gameplay. 25 in the tower isn't great, but if you are getting 25 also while fighting, and it doesn't seem slow or laggy to you, then 25 is good enough. But thats getting down close to the minimum, in my experience.
What graphics/video settings do you have on, or what are they set at? If you turn down the texture size in the video settings, for example, it will help with your frame rates. Play with the other settings as well, you may be able to increase that frame rate without upgrading anything. Although, this will lower the amount of eye candy, its usually a worthwhile trade off.