Upgrade your graphic card for sure since that is the main component for video games

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Your RAM is fine, but if you want to upgrade go ahead, they are relatively cheap now (I've seen some as low as 20$/8gb after rebates)
Something else you might want to consider, your hard drive. If you want better performance (less lag, etc) then I'd recommend upgrading that after your video card. If you're running 5.2-5.9k RPM, then I recommend upgrading to 7.2-10K RPM for sure. The BEST upgrade for speed is getting an SSD, most are 5-10x faster than normal hard drives!