However, these are only any good if you have a wealth of keyboards attached to your computer as they don't actually make any sound.
Not true.
VST instruments are fantastic (instruments that run as plugins in Cubase or other host sequencers). They are as good as, if not better than hardware instruments and they exist solely in the digital domain of your PC (so no signal to noise ratio to worry about, plenty of memory and instant parameter recall). They are also much cheaper. You should probably start with a sampler as these can 'sample' and reproduce pretty much any sound, real or otherwise.
I reccomend the Halion sampler plugin, or Kontact if you're well versed in sampler tech.
Cubase is quite frankly the best thing in music/sound creation. I have it coupled with Propellerhead Reason, which can feed its audio directly into Cubase via an invisible system called ReWire.
When you stop and think about it, a sampler/synthesiser is just a highly specialised computer with inputs/outputs. Modern PCs blow the processing power of these things away by a huge margin, and editing on a computer monitor is a damn site easier than editing on a tiny LCD screen (refered to as 'trying to decorate a hall through a letterbox!).
I payed £1000 for an E-mu sampler, and £200 for Halion. Guess which one I used more?