1) It sounds like you need to build a new system if you purchased low-end parts 3-years ago. If you need help then post a price range and expectation (ie if you need a new OS, monitor, mouse/keyboard, speakers, or just bare-bones tower etc..).
2) Yes the faster ram will be clocked down to the speed of your slower RAM. Think of your RAM as a white-board and the clock speed as the passage of time...it would be weird if you were writing down the middle and the writing on the left half was appearing slower than the writing on the right half. And potentially someone could miss-read what was wrote..etc...Thats the basic idea of whats going on anyways.








