Assuming you don't want to spend an arm and a leg, you're pretty much stuck with what you already have.
Graphics? Can't upgrade. Well, technically you could but the price can't be justified considering the cost of the laptop itself
HDD? It's already 160GB so not that great a need for space.
CPU? Ain't gonna happen.
About the best kind of upgrade you can hope for is to max out the RAM.
Even then, trying to play Doom 3 on a laptop with an Intel GMA 4500M solution... well, as long as you're not too crazy about eye candy.