I've got that same stupid built-in card in mine. Don't forget to factor in the price of a power supply if your current build has less than a 500W. I was thinking of getting a GeForce 9800GT as an upgrade, but I'm no expert.
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Sorry to burst your bubble but your going to need something stronger then even my HD4890 to fully max out STO with no hicups. I'd look into a 750Watt PSU and an HD7870 or GTX660Ti, that'll set you up for a good while. An I mention the PSU cause in a pre-made PC they always max out the PSU regardless of what is in the PC to save money.