hmm cant i buy a 4gb ram and install it with my old one i have two ram slots left
You can as long as they are using the same voltage, and you may have to set the timings so they match and don't conflict. I prefer buying ram in a pair to avoid issues.
As long as you weren't someone who cared about AA+AF I would say a 660Ti would fit the ticket the 7870 would as well but 660Ti is comparable to the 7950 http://techreport.com/review/23981/radeon-hd-7950-vs-geforce-gtx-660-ti-revisited
It's really up to you the 660Ti is like a card on the edge of being high end and the 7870 much the same but more on the mid range of cards. You may be able to find some 7950's in your range but they may not be the best quality of card. And your card is quite hot yea I would replace it as well.
But before I go to far into this thread what power supply are you currently using to power your system.