you could always spend more money on better synth modules/samplers. Even go try some software synths. I would highly recommend doing this if you are serious about your MIDI projects. The built in sounds of the Yamaha are good, but really aren't meant for professional or serious users. Buying a separate module would increase the quality of your work and creativity greatly. Then you could buy effects processors or plugins for Cakewalk to add effects to your sounds. If you find Cakewalk doesn't have enough MIDI editing options or you don't like it, you could try some other kinds of software....Logic is a good one.
Also, if you plan on bouncing your MIDI tracks down to audio, you'll need some way to get the audio of your synth or any other synths you buy back into your computer. You could probably find a fairly inexpensive soundcard which could do this for you. Look for one that has a line level input along with MIDI input. Then you could just scrap the USB idea and keep it all on one sound card. Just search around the forums for a good soundcard to help you. Hope this helps, good luck.
Again, it all depends on how much money you want to spend. If you're REALLY serious about it you can spend $1,000 on up for a good module. Kurzweil, Roland, or E-Mu all make really decent ones. Look around more in the MIDI section of these forums for more ideas. Many of the keyboard people in there know more about this stuff than I do. Also, just do a search for modules on sweetwater.com, samash.com, etc. Just beware, MIDI can be a confusing new language to learn. There are so many functions that can be done with it, but once you learn then it can help you to become extremely creative. I'd also suggest getting a MIDI reference book to start understanding more about what you can do with it. Hope some of this helped, good luck.
And you can just go to Amazon.com and search for MIDI and a bunch of book will pop up. I've never read any of these though so I'm not sure how good they are. Best way is just to go to your local music store and browse through a few books to see if it sounds like it's easy to understand.