what type of sounds do you like? ware you more symnth oriented, or acoustic oriented? you want pianos, strings, guitars, flutes, organs? or leads, pads, etc....?
keep that 550, and use it as a midi controller and save for....
motif rack, $1000 new/$850 on ebay
WHY? best all around pallette of all the major modules/boards. it has the BEST solo violin/cello/viola i have EVER heard from a module or keyboard. solid acoustic drums. great rhodes/wurly sounds as wel as solid organs..... solid in every category and only suffers in the pads department (as compared to others) and has very flexible synth paremters, plus it comes with editin and sequencing software. expandability sucks, however.
OR....
EmuProtes 2000
New- $600, ebay- $3-400
WHY? cheaper and the most easily expandable and has a very useable pallett e f sounds. FOR THE MONEY, the best all around buy, but it gets old very quick. some very good organs on here... solid hip ho pdrums; acoustic drums suffer.
Roland 5050
new, $800. ebay- $650-750
WHY?
the best strings and orchestral sounds and the best expansion options, by far. the best pianos as well, and the acoustic drums are very good. as always, pads, leads, and atmospheric sounds are well represented as well. i have played with, but never owned this one.
these are your sound module options. the triton rack is about 1500 and to be honest it isnt worth what it costs when there are so many better, more up-to-date modules on the market for half the price.
TRITOn...
the triton le does not rival the fantom or motif in the "traditional" instrument categries, but its pads and leads are exceptional. the triton le also limits your options by its limit of one insert effect, as opposed to the 5 on the motif rack. you also have to create your own sounds in the sequencer mode, so the sounds you loved in the program mode wont be there in the same way. when you only have one insert effect, this will give you problems, unless you have a rack full of outboard effects modules to work with....
also, take sequencing in consideration: what will you be using as a sequencer? what type of outboard mixing and effects do you have? what are you recording to?
i am speaking from experience here; i have owned, in the last year and a half, the roland xp 30 and xp 80; the triton le, the emu proteus 2000, the korg tr rack, the motif rack, the mpc 2000, and now cubase sx and sample tank 2. i am by no means an expert and these observations are made based on my tastes and opinions, and so they are purely subjective..... but i came to know what each is capable of and some of its limitations, as well as ways qaround those limitations.
in the end, you need to get 1- something that you will use for the long haul, 2- something that meets your current needs for your current setup, and 3- something you like! gp check out everything you are looking into yourself. play with them at guitar center or sam ash and get a feel for the sounds and features. i hope this gives you some insight.
OR.....
you can get the fantom, triton le, or motif- all three of which are simply keyboard versions of the modules with differing pallettes on some and different polyphony issues on others. the keyboards cost much more money, so rememebr that. actually, you can get the fantom (original) for around 1299 some places, which is still less than the triton rack.