check the psu wattage. Just open the case and stare at the psu
, u should be able to spot the wattage number.
New vid cards need quite much power, and usually the prebuilt computers don't come with good psus. The current vid card uses pci-e so any pci-e card should fit into the mobo, u could grab a 7900GT, the price/performance ratio is great!
And about the ram, 1gb of ddr400 is enough to run most games on high settings. Some games like battlefield 2 will require 1.5gb (if u are getting more than 1gb then take 2gb) to run smoothly on high settings. So u should be fine with 1gb, but the 2nd gb will help, and all loading times on maps etc will be much faster.
So if u can afford it the second gb would be nice, but it's not necessary.