Buying a whole used computer might give you better return on investment.
lehtv's recipe sounds good considering that this seems like a difficult budget. You have to sink a good amount of money to get something solid that works, but you don't have much left for performance. Spending just a bit more would be a good idea, IMO. In particular I'd try to cram in a decently powerful CPU like the i5-2500k, so that when you upgrade the GPU in the future, there's no need to up CPU at the same time. As is, you'll probably have to upgrade both if you upgrade one, because the other will bottleneck so badly.
Try to look for a cheap used display. That's one place where you should be easily able to save.