After looking at your setup and considering your budget, my advice is

1) Don't replace anything that's working if you're happy with it.
2) Check out Reaper DAW software ($50.00 shareware, updates are free... and it easily rivals the top name products.)
3) Garriton Personal Orchestra and/or Jazz Big Band sample libraries if you want to upgrade midi sounds. ($200.00 each and worth every penny.)

A second (audio only) hard drive is a good idea. There are a number of inexpensive USB audio interfaces for getting your audio into the computer. Perhaps someone else can recommend one of these for you.

All of the above can be done for about $1,000.00.

Have fun,
Dean