I am no expert but I think what you have already is suitable for a basic home recording studio. I might invest in a large external hard drive say 300gb, ideal for dumping recordings to as they take up so much storage space and you will use up the 200gb or so you have already very quickly which will slow your pc down. Cubase is perfect for your software needs but you may want an interface if recording multi channels at the same time. You may also need to upgrade your soundcard. Speakers are important but I would put replacing your Creative on the back burner till you get the other stuff up and running properly.
Best of luck JIM