Your onboard video card is not up to the job - 8MBs is very low these days, I would suggest a separate video card of 32MBs as the minimum investment, there are some fairly reasonable ones about now as most folk are fitting 64MB and 128MB vid cards. Quite often on-board video mother boards don't have AGP slots, if yours has an AGP slot available get an AGP one, if not get a PCI board and disable the on-board one in bios.
This would give an improvement but your CPU is marginal also, and most fps increases come from a faster CPU feeding an adequate video card. The CPU speed has more influence than the video board as long as the 3d vid card is reasonable. It wouldn't be worth spending too much dosh on a super video card without upgrading the CPU as well I'm afraid.