i am having problems rebooting my mid 2009 macbook pro (grey screen other issues), need help!

I am having problems rebooting, I have experienced the same problem as others with the grey screen but my case is a little different, It started with my mid 2009 macbook pro acting a little slower than normal, the screen would freeze, mouse wouldnt move and i'd be forced to "force reboot" this went on for a couple of days until i was tinkering with the wifi connection and it froze again. I had to reboot and this time i noticed that it took a long time to get to the login screen. i put in my password and it went into infinite load (i also tried the guest account and other accounts with same result). i tried rebooting again and the same thing would happen. after several reboots, the screen just stayed grey, sometimes with the apple logo and gear sometimes without it. Since its a 2009, (and for the most part, still works like a champ) i decided test the ram, i took one stick of ram out at a time to see if i could notice anything wrong with them. when i stuck just one in, the mac was still having the same problem rebooting. i pulled that ram and and stuck in the other and i noticed that this time the apple logo never appeard and a big crossed out circle appeared. figuring that the ram must be the problem, i ordered new ram and installed it. i noticed that when i rebooted for the first time with the new ram it was rather quick and took me straight to the login screen and i was able to log in with no problem. at this point i thought the problem was solved, but i noticed the macbook still acting funny. at first, none of my files appeared on my desktop or in the finder window. after about 20 minutes, everything magically popped up. i also noticed long pauses/freezes with the colored pin wheel spinnig. i closed the laptop and opened it again and the screen just stayed black. i tried rebooting again, and again i got the grey screen with no signs of being able to log in... i waited about 20 minutes and tried to reboot again and again i was able to log in, however the macbook is still acting slow and will freeze with no signs of unfreezing without a reboot.

my macbook is 5 years old and has plenty of miles on it, so i understand if the thing needs to be replaced alltogether but does anyone know what could be the issue? could the bad ram have corrupted the hard drive in anyway? would re-installing the OS fix it?? and if so, how would i do that if i downloaded mavericks from the app store? are there any other solutions to this problem?

all my files and what not are backed up on an external HD so theres nothing on it that i'd miss desperately if i had to whipe the hard drive but id really like to get her working again without any whiping of the HD. if anyone can help me out, it'd be greatly appreciated