I've got a 90 RG550 and a 91 (serial with 92 spec) RG770DX.
They're very similar guitars, the 770 has the fancier image, and is just as playable. The only things I prefer about the 550 is that mine has an original Edge trem as opposed to the Lo-Pro on the 770, and that my 550 feels slightly more fluid to play.
I've kept the pickups from the 770 too (which are colour matched from stock) as they sound pretty damn good as generic rock pickups
the ones in the 550 had to go.
Either is a great guitar. I'd not buy an RG550 for much more, or much less than £400, and an RG770DX I'd expect to pay a bit more for. (assuming both are in ok condition and given their age).