A) you would be suprised at what people expect for a budget. yes. i assume you are in japan (according to what you said).
B) again... we have no way of knowing what your budget is so you must tell us. what i think of as mid range may differ from what you do. we need an actual figure to work from. for example $400 or ¥40500.
C) the z906 is not quite as good as the z5500 but its not bad. you could move the z5500 over and get new speakers. you could even hook up the pc and tv and ps3 to one system if they are reasonably close to one another.
D) there are a few 7.1 computer speaker sets however the majority are 5.1. since there is a lack of choices and since there is a lack of content for 7.1 i just wanted to verify that you knew this and wanted to proceed regardless. the gigaworks s750 is a 7.1 set for pc use. there are not a whole lot of them however. listing 7.1 is mostly a marketing gimmick since not many people will use it.
E) i agree.
F) then going with a z906 or moving the z5500 would make sense. you could always upgrade your tv setup with something more "audiophile" eg a set of home theater speakers and a receiver.
if you re-read my original post i did suggest that you might want to look at home theater speakers provided you have the budget for it. its what i use for my own personal system.. i use a klipsch satelite speaker set and sub with a pioneer receiver. it works out great.
anyways.... its completely up to you which route you want to go but your options are limited to what your budget is.
if you want to upgrade your hdtv system speakers and move the z5500 to the pc we can help you pick out new speakers
if you want to upgrade to a home theater speaker kit for the pc we can do that.
if you want to just buy a z906 perhaps there is a cheaper place for you to do it (do you live near big city tokyo.. akihabara has tons of electronics..)
i've even seen a few home theater kits which accept rca inputs for each channel (front left, front right, center, sub, rear left and rear right) which you then use 3.5mm to rca splitters on for use with a soundcard. while this isnt the route i would choose to go myself... i've seen people on here discussing such sets.
you have to let us know what you were thinking in order for us to give you any consultation.