I'm not suggesting you have a weak, or slow system. The Athlon II is a bit "dated", but still capable of running your games. I am jus' saying there may be other things, (like slow internet, or full HDD) that might be slowing it down, (lag).
I've found that to be the case a lot of times.
If you have the $$, and want to build another system yourself, then I would start fresh, with a new Intel Ivy bridge platform, (Intel jus' soundly beats AMD in performance these days) and add on, according to how much you can stand to spend.