The highest video card I know of is the ATI 5450 that you can fit without changing the PSU. When it comes to gaming, I see folk try pushing up the CPU but in this case, it's not the CPU. I don't know how many PCs you've had in the past but that 210 is only good enough to let the game play. Not play well.
i have a Dell OptiPlex 330 MT. It has intel e2160. which is a dual core that pushes 1.8ghz. im really not sure if i need to upgrade, that seems quite slow to me...but i kno diablo 3 and modern warfare 3 both ran, online, with little to no problems. i also have 4gb of ram and an asus nvidia en210 1gb video card. as far as i kno my ram is maxed for my computer. and i really have no need for a better graphics card. but id rly like to change the processor. my comp is great if im only doing one or 2 things, but if i start doin more than a handful it slows WAY down. i kno task manager shows CPU usage at 100% a lot.