Well.......framerate and lag and not necessary the same thing. You may get great framerates but lag badly due to bad internet connection. The trick is trying to find your framerates and see if they're good or if they drop down really low. If so, that may be the prob. If not, then the lag could be caused by 2 other possibilities off the top of my head:

1. internet connection

2. CPU resources. Let me explain: I had a problem quite some time ago where I would lag badly every 10 seconds or so in game. I finally looked at my application/CPU processes and saw that it would spike very quickly every 5-10 seconds, then come back down. This coincided with the lag. So the lag was actually not internet or graphics card, it was a little app that was causing a brief but very noticeable resources usage spike for my dual core CPU. That was the prob. It's not something you'd normally think of, which is why I mention it.

I'd suggest nailing those down. Find your framerates first. Use FRAPS if not availabe in-game. Then check your CPU/resource usage and monitor it. You may need to turn off some background resources and apps, there's a chance that's a problem.