Thank you for such a good response.
I have only 4GB RAM and that's a problem, I know it. Every time I play SWTOR I end up with swapping out of RAM. But the problem is I run my Core 2 Duo on 2.89GHz overclocked with Corsair Xtreme RAM modules all those 6 years and now 800Mhz RAM modules suitable for that overclocking are quite expensive in these days. I tried some cheap Kingstons, but overclocking always failed. So I realized I don't want to invest in expensive slow 800MHz RAM modules.
But maybe you are right, that GPU must come the first. The question is how much my old Core 2 Duo will slow down fast Nvidia 660 Ti card.