What Should I Upgrade In My Computer?

I have Nvidia Geforce 8600 GT ( approx total memory 981 MB ) Processor: Intel(R) Pentium(R) 4 CPU 3.40GHz (2 CPUs), ~3.4GHz Memory: 1536MB RAM Available OS Memory: 1470MB RAM Display Memory: 981 MB Dedicated Memory: 501 MB Shared Memory: 479 MB idk how to check the name of the ram iam using but i know how tell me ( i use windows 7 ultimate ) if i missed any other info tell me and i will get it sorry iam not computer professional