When do you experience the "lag"? As soon as the game starts? 20minutes in? All the time? At random intervals?
Is your lag a stutter, or extended periods of lower fps?
What OS are you running?
What videocard did you replace?
What is your PSU?
When was the last time you defragged your HDD, or performed a clean install of your OS?
There can be few quite a few causes to investigate when diagnosing underperforming hardware. We need more information before anyone can give an earnest diagnosis or suggestion.
Do you experience lag while playing online only? what if you play single player? Do you experience it then? How are you connecting to the internet and what speed? Also, how do you connect to your router or modem? cable or wireless?
That is stutter. In my experience, stutter is generally related to system or graphics memory, or hard drive transfer rates. I think a wait and see approach with the memory ugprade would be the most efficient route to take. No point in metaphorically ripping apart your computer if the memory upgrade solves your ills.