how do you determine what to upgrade next?

i have a relatively inexpensive system that i like quite a bit, but am also always under the spell of wanting to upgrade something. I'm not the kind of person that will (or even has the ability) to throw down thousands of dollars on a complete overhaul, or one piece of gear, I'm more in the range of 200 to 400 here, 40 bucks on a used cable there. probably would not spend more than $500 on any one thing, and am always looking in the used market.

I know there are a lot of different ideas about what the most important part of your system is, and am certainly not looking for "the answer", but am throwing out a general query on where i should invest next just to get some opinions. (and I'm posting this under speakers because i think that's probably the answer). I really anjoy listening to what i have, but always see room to make it a little better. note, there is no cd player, so aside from listening to NPR through my old sansui receiver the TT is my only source. I *might* buy a cd player sometime, just for convenience sake, but won't spend a lot of money on it.

here's what i got:
Music Hall mmf2 with a clearaudio aurum classic
forte f-40
adcom gfa 535 (mk1 i think)
paradigm 5se MkI
this is all in a pretty small room, maybe 13 x 12?