Tracer what you have is a very nice "soccer mom" computer that will do what 99% of its buyers expect it to do. What it wasn't designed for is graphics intense gaming...hence the integrated graphics card. Obviousely you know how far you can push the AH graphic settings. I'd suggest you try to live with that. You admit to being in the dark when it comes to computers. As others point out, to boost graphics you'll need a dedicated grapx card, which in turn will require a more powerful power supply. Those two componants alone will run you about $300. Does your emachine have a PCI slot for the card? It may or may not. Will the new power supply be compatable? It may or may not.
My .02 cents, save some money and build your own gaming rig or have someone build you one. Rich (TD Computers) has built quite a few for AH players and will work within a reasonable budget. Modding out your emachine is risky and not very effecient from a cost point of view. Think of throwing a Hemi into a Prius
I built a desktop a few years back for less than $1k (including Windows OS) and it still max's out a frame rate of 60 at peak AH graphx settings.