If you have onboard Graphics and it sounds like you have, they cannot be disabled very often in BIOS like Sound can but it should not be a problem. Your computer should work better with a stand-alone card. Your CPU should not be doing the work now it's the shared memory that's the problem, as the onboard Graphics sound chip should do the work
a good graphics card will be able to take some of the work that the cpu does ,for instance T&L can be done by some graphics chips whereas some cant thereby the cpu has to do the work, or with some games they cant be played at all so if you get a better graphics card you wil probably start working your whole system much harder because you will be able to do more . but as the lads suggest the real benefit will come from the dedicated video ram.
just six months ago the magazines were raving about MX440 chips now they are only about £35 so buy a card and try it out. I would have to disagree with woodchip about disabling the onboard graphics in the bios, its simple if the mobo has an AGP slot then the onboard graphics can definitely be disabled in the BIOS.