NVIDIA GeForce GTS240 1024MB GDDR3 and possible replacements

So I'm thinking about buying a new PC and I'm thinking of getting a Dell Studio XPS 9000. It's had ok reviews, but largely from people who seem to be reviewing a different configuration than I'm going to be using. The reason I'm buying a Dell is because I can get it pre-made with support and pay it off over a year due to a particular arrangement they have in Singapore.

It all looks ok to me except the graphics card which is a NVIDIA GeForce GTS240 1024MB GDDR3. I'm looking for some opinions on this card and also to see how easy it would be to change the card after purchase if it starts to slow down. I'm not really playing high end FPS games, more RPGs and strategy games, but I like to eb able to break an FPS out in the future...