Do your friends also have a low powered CPU? LoL, Minecraft and similar games generally rely on high single threat throughput, which is something that AMD CPUs generally are very poor at.
One test you could try is turn play the game with your current settings with no antialiasing and record your frame times and min/max/average frames with something like FRAPS. Play some rounds and average the results. Then do the same thing with AA stuff cranked way up, make sure you do teh same numebr of rounds at least, but do more than just a few since multiplayer games change every round so you want enough to gte a good average rather than just one or two where whatever happens in teh game could affect the results more than the AA settings.
If you notice that the AA rounds the Min/Max/average all drop by a similar percentage (say they all drop by 25%) you can pretty nmuch say that your video card is the cause of the slow downs. If however the drop happens to only your max/average, but the min stays pretty much teh same then you can conclude that during times where your frame rate is slow that something else is causing the problem (which is likely your CPU).