What do I need to upgrade on my pc to run games more efficiently?

My first build was not intended for gaming, but I have changed my mind and would like to squeeze max settings out of my games. I'm running a cheap A74ML-K foxconn mobo w/ 4gigs of mushkin ddr2 ram until my ASUS M4A79XTD EVO AM3 AMD 790X ATX AMD mobo arrives in which I will use my 4gigs Gskillz ripjaws ddr3 ram. Will those 2 upgrades make a big difference in gaming and if so, how? The other specs; AMD phenom x2 processor, Nvidia Geforce GTS 450 w/ 1g dedicated. I know the graphics card processes pictures/graphics for the CPU so does that translate into faster load times? My main problem is when I'm playing Civ 5 and it shuts down and says something has caused Civ 5 to stop working. Also it has frozen several times, more so the farther I advance in the game. I can't even win a time victory because it freezes or won't load everything. I've brought down all the settings to medium and haven't had a complete freeze out yet. Please help.