This is a tough choice because in this case, really you should do both. If you can only afford to buy more RAM or a new GPU then the best decision depends on your existing GPU. If your existing GPU has at least 2GB video memory then it's better to buy more system RAM. However if your GPU has only 1GB or 1.5GB video memory, or is a integrated GPU that shares your system memory than a new GPU is the best decision. Either way, whichever one you don't upgrade right away should be your next upgrade.