Is it worth upgrading my PC?

I have had my PC for a while but it isn't good at gaming and there are a lot of indie  and A A A games coming out that I want to play. I am not sure how powerful the power supply is I can't find It on the Packard Bell website.

Current PC specs

I5 650


AMD Radeon 5450

Windows 7 64bit

What I am thinking of upgrading is the graphics card to a GTX 750TI and add 8GB RAM. Is it worth upgrading to play games like titanfall,  Assetto Corsa, DayZ and dark souls 2? I have a really low resolution monitor and need a new one what resolution would you recommend for the upgraded version of my PC? 

I said add 8GB RAM but I meant to say upgrade the RAM to 8GB.

750 ti is a good upgrade but you should check your power supply. You should find out by opening your pc case and look at the amps on the 12V rails