I do recommend Cakewalk. Hopefully, someone will create Cakewalk instrument definitions for the PSR 2000 soon. If no one else does it, I will.

You don't have to spend a real lot of money, but I recommend Pro Audio 9 at least if you're going to record audio. Buy it, read the manual in your spare time.

What I do for songwriting is I find a style that comes closest to the song idea I have. I record the chord progression and style onto Cakewalk. Then I edit. Do I want the drum roll from another style or another midi file? I copy and paste it into the measure. Is there a special break or build up that I couldn't express through the prefab Yamaha style, then I play it or paint it with my mouse in the appropriate measures. Do I have a better bass idea? I punch it in where I want it. Do I want the song to slow down towards the end? I go to the tempo view in Cakewalk, and with the stroke of my mouse, the song slows down.
I use Cakewalk to select other instruments because with a big computer screen, you can see a lot more of what's available inside the PSR 2000. You can select other channels on your computer screen to insert extra parts.
You need to decide how far you want to go to get what you want. Maybe your song fits perfectly with a style. But if one style fits better for the verse, and another fits better for the chorus, then it's great to use Cakewalk. You can record the verse on one file. REcord the chorus on another file and paste them into the same file and do some editing to make it all smoothe.
REmember when you transpose with Cakewalk, don't transpose your drums! It will sound horrible!

Good luck,

Larry