How do i improve gaming performance?

I am tryin to play warcraft III on my PC. It runs perfectly smooth in the start and as time pes it gets sluggish. Any comments on this. My hardware specifications match that of warcraft 3. I dont use a graphic card coz i cant afford it at the moment. I have 768 MB of ram and is shared with the video ram which is 128MB. My AGP Aperture Size is set to 512MB and page file memory is 2GB. I tried many programs that say that it can increase the performance but it doesnt even help a bit. Please help.