memory

I have a dell xps 9100 desktop with problems. Lately, I have been getting freezes, blue screens and system crashes constantly and cannot find any solution to my issues. The computer may run a program like auto cad and work fine but if i go online and play a game it does this weird sound and crash continuously after a game play or two. I did a diagnostic check on it and when it reaches a certain point it crashes. I may have noticed that it would be the memory portion and may need more but how do i find out if the memory is one not enough when it says i have 8GB and only used 16% of it. Or two, how do i check if its only one or two cards that are bad and not all of them? I have called Dell enough times to bother them but it seems that they dont have a geek squad to figure the system out. Since the product has no warrantee, they are reluctant to help me, well that is what it seems to me. Any help will be appreciated. GE