What's your screen resolution? What types of games do you play? FPS, RPG, SIMs... or all of the above?
Yes, that Momentus XT would work in your system and that 4GB flash storage makes it faster than most HDDs.
But do you already own a HDD? At $110 that Momentus XT is more expensive (and slower) than a full on SSD like the OCZ Vertex 2 40GB SSD @ $105