Re: Ibanez Rg 550 vs 1570 vs 2550 or
The guitars you mention are all relatively similar.
550 has a pickguard
570(1570-what they call that model now...same goes for the others, Ibanez puts a 1 or 2 in front of the old #'s now) does not have a pickguard
2550-fancy inlays, upgrade pickups
They're all great guitars.
Older 570's for some reason tend to be priced a tad bit lower than 550's, so there's some real bargains out there.
If you like a really thin neck, the older ones are the way to go. Otherwise, if you want the 17mm at the first fret measurement neck like the older Japanese RG's, you are in $1000+ territory.
Prestige is the current reference for the Japanese (high end) guitars that Ibanez is building now.
The Ibanez website has all the specs on the guitars.
Oh and regarding the pickups...try the ones that come in your new guitar for a while. You may be pleasantly surprised. Most of the RG's you seem interested in are made of basswood. Evolutions come stock in the 7vwh which is alder. Evo's are awesome pickups but also reveal any flaw in your playing technique and are rather bright sounding. Some here on the forum may call me an oddball, but I've never had a major complaint with the V series pickups that come stock in a ton of the Ibanez guitars. The INF pickups in the cheaper imports on the other hand, are lacking in a lot of areas.
Hope this helps, although a lot of this is vague and will really boil down to your matter of preference. Try out some of the different prestige guitars offered today and seek out ways to try out an old 550 or 570 and see what feels and sounds best to you.