If you're not planning on replacing the entire system and just want a speed boost and reliability increase with large scenes get more ram if your mobo will allow you to. Large scenes eat up ram and if you hit the pagefile your HD will be swapping and you'll lose a large chunk of performance unless you have an SSD.

Try lowering your preview texture resolution to 256 or 128 from the default 512 if your preview window is acting very laggy.