I've always beleived music can be good even if it doesn't sound real. However, this would be better if it did! Have you considered buying a newer keyboard? The Casio WK- 3800 is pretty much a good balnce of sound, functionality, size, and price.
If you aren't willing to purchase a brand new keyboard, than tgsf21x (google it, it's free) is THE best GM- soundbank I have come across so far (I use it in most audio I make). If you are going for orchestral, just combine that with RolandOrchestralRhythm and you are set. If you are looking for a professional soundbank without all the price of a new keyboard, than go for Garritan Personal Orchestra (I think it's about $200 to purchase it).
Oh, and what keyboard are you using now?
P.S. Please Check out my Castle Crashers Submissions, too!