Don't waste your cash on more memory. I am still gaming on 4gb. So far the games that claim to need more memory are running fine.
Since eventually you will upgrade the cpu+board+memory. So going with a faster gpu is not a waste if you can reuse it later on.
Because it's likely you don't have a large power supply. I would pick the low wattage RX480 or GTX1060 if your buying new.
A used GTX660, 660ti, 670 and HD-7850, 7870, R7-265, 270, 270x and 370 all are easy on power too.
If your power supply is ok, maybe a cheap used card like a $20-$30 HD-4870 or GTX280 would be ok. A $30-$40 HD-5870 or something like that would be ok too.
Running winxp or win7 32bit? Most newer games now need win7 64bit and higher to run but most older games run fine on those older cards in
win xp running DX9.0C.
A used GTX680, 770, are at least as fast as the GTX960. They will run very fast in winxp 9.0C but can also run DX11.
A HD-7970, R9 280x or if it's cheaper a HD-7850, 7870, 7950, R9 280, R7 265, 270, 270x Geforce GTX660, 660ti, 670,760ti, would be ok too.
Look at gaming benchmarks so you know where the card places.