It looks to me like you have a very nice system with good balance between components already.  When in doubt, I usually like to upgrade the source to make sure I'm starting with the best signal I can afford.  But your CD player is not a bad one, so you'd probably have to sell what you have, and spend all of your budget and maybe more, to get significantly better sound.  Same goes for your speakers, which are the other thing I like to upgrade, all else being equal.

In your situation I don't see a clear path just from looking at the equipment. Seems to me it really comes down to what you personally think is missing in your listening experience at this point. Your listening environment, along with your personal preferences and goals should lead in the right direction. Do you want more dynamics, better clarity, warmth, detail, etc...