NVIDIA® GeForce® 6150SE Graphics
PCI Express® x 16 slot
You can get any PCI-e x16 GPU, like a 6870 or a 560, just make sure the card isn't too long for the case and that you have 6pin power on your PSU to power it.
Now the bad news. You only have a 220W power supply and that's not going to drive crap! You will also have to change the PSU to at least 450-500W to use a new GPU.http://support.gateway.com/emachines/desktop/2010/emachines/et/ET1352G/ET1352Gsp2.shtml