Nick K says:

You have a pretty good CPU already. Assuming that you don't have an old graphic card in your a PCI Express x16 slot, the cheapest option for you is to buy a new graphic card and install it in there(I asked because Gateway doesn't say if there is a stand alone graphic card installed). You'll also want to upgrade your memory to the max 2GB limit, it'll help tremendously for today's power and memory hungry games. In total, it'll probably cost you 60 bucks for a graphic card that can run D3 plus 20 bucks for 2GB of ram(maybe less). I don't know your budget, but you definitely can put some more juice into the graphic card to achieve better effects. Take notice that this is only a temporary solution, a year to 2 years tops, eventually your system will need to be upgraded. Buying a new computer now has the advantage of having more upgrade possibilities in the future, since your motherboard doesn't support or have the things newer hardwares need.

Remember to check compatibility to your motherboard before you purchase new graphic cards and RAM.