Okay, i've been there. I mean, in a position where i go check with system requirements lab website every time. It's my personal opinion.. but, i think the site is a pile of bullsh!te. It just takes the requirements posted by developers and compares it with your system specs. No real world experiences are taken into account. In short.. it is for the people who can't read the Readme.txt file in the game and the MSR posted in their website. I didn't knew it until i realized it. which was nearly 2 years ago.
My serious suggestions is to go for youtube. There are hundreds of videos that feature gaming on G31 chipset. Go check the performance in their rig... Most of them will post FPS count. Relate it to your PC specs.. and take the decision. Don't always rely on MSR. Most of the games can run in the configs that are far below the MSR. Just go to youtube and search for something like... "Left4Dead 2 GMA3100" and you'll get atleast 2-3 videos featuring your GPU. And.. there is always this nasty thing called "Demo" which we are not witnessing that often these days. But again.. if you have the chance to grab a demo, grab it. And check the performance for yourself.