To tell you the truth, I would minimize my investment doing the upgrade--basically, get it running so you're able to do what you need to do, spend the minimum. But 7-series GPUs are running pretty cheap by now, so you're not in any trouble there.
This is the time to begin reading up and saving money to get started on your first build-it-yourself PC. $700-$1000 will get you a very, very good computer, if you're willing to take the time to learn all you need.