I think you are on the right track. Use what you have first, and buy what you don't have. You have a sound card and it sounds like your keyboard will allow midi input/output, but what you don't have is a good quality DAW. So Sonar is a good starting point (I have Sonar X3 Studio - a nice upgrade from HomeStudio 6). You can also look for a soundcard driver called ASIO4ALL. Not as popular as it use to be, but if the current sound card drivers don't work well, this may be worth a try. And you can use your keyboard to input midi notes into SONAR, then play around with the built-in sounds in Sonar and play the composition back through the soundcard (not the keyboard). As time goes on you will realize what you want next and can add it into the system - and trust me, there is always just 'one more thing' you will want to get!
 
Also, don't forget to check out the free stuff on the net. There are some really nice gems out there