What are you wanting to do, that you are not able to do now? Extreme gaming seems the only thing this machine cannot do.
Without an AGP card port, you are limited to a PCI card or the onboard video. When this card is added,the onboard video will be disabled, or you may need to turn it off in the BIOS.
It looks like it came with a gig of RAM installed. More will probably not be helpful for usual computer tasks, but you could add more as you mentioned, assuming it is the correct kind.
It's amazing that a high end machine does not have an AGP or PCIe slot. What onboard graphics is in there? You can right click on Display Adapter in Control Panel/System/Device Manager to see. I am not sure a PCI card will out do onboard AGP.