Dont kill me, if im wrong, but i dont think there is a white floral JEM out there..! It was propably a JEM777WH and if it was and the price was 850$, you should continue kicking yourself!
There is not really difference between the RG770 and the RG550. If they are older versions, the RG770 will have propably a Lo-Pro Edge floating bridge, while the RG550 will come with an original Edge bridge. Look up, what are the differences, but the Lo-Pro Edge will be more comfy in my opinion. The other differences will be optical only. Same neck, same playability, same body, (depending on pickups) same tone! Both and any other MIJ Ibanez from RG-420 will be a lot nicer feeling in your hands and providing better sound than the RG350! Its experience of mine, most people will agree with me. So if you liked the RG350, dont hesitate much, rip a MIJ from ebay, or elsewhere!!
Before you go for the Prestige series, get 1 of these, coz: Lower end Prestige ones (1550,1570,2550,2570, etc) wont give you more satisfaction the those MIJ´s we have been discussing about. They´ll have the same tone (if not worse with the fukkin Ibz/Dimarzio pups) and playability, but you´ll pay more.. They are a bit more shiny, but personally i prefer nowdays the ones with a bit vintage looks. If you go to higher Prestige and J-Customs, they have mostly mahogany, or rarely ash bodies, they´ll give you a different tone caracter, than basswood. Dont jump there yet, if you are not sure, you like the mahogany tone, or not! They are very expensive, so if you are into basswood sound, there is no point spend your money on them..
Welcome on the forum and good luck choosing the right baby! Hope the answere helped!