Well consider this, before i got my new 7600GT, i had a 6500...which is basically the same as the onboard 6100....and i could hardly run any newer game, from San Andreas to Battlefield 2...once i got the 7600GT, i can run nearly all games at a reasonable resoultion and graphics setting, with a good FPS. It sure is worth the upgrade, i highly recommend it...If you have more money to spend, go for either a 7950..or even more, an 8800...coming from an onboard 6100 you won't be dissapointed..
Matt