I've been playing Istanbuls for a few years, the Agop Trad Darks are my favourites by a long way. I too have a 16" dark crash which is beautiful.
I added the 20" dark ride - it's an absolute beauty, for me it treads the line between clarity and explosive-yet-subtle crashiness perfectly. Obviously it takes a while to learn how to control it, but when you and it are on the same page it's an extremely rewarding cymbal to play. Very expressive.
I'd recommend an 18" dark crash too, my all time favourite cymbal (so far). For me, the 16" hints at what lies within the 18", it's got everything - perfect attack and decay but the smokiness lends it great character. I can't walk past that cymbal without tapping it, just for a quick fix. These are, of course, all things I like in a cymbal and may not be to everyone's taste.
As for how they hold up, I must admit I've been through a couple of 18" darks. I used to play in a very heavy band and thought I'd risk it, but they're a touch too delicate to really go hammering them night-after-night. They'll take a beating, but if you're going to lose it and thrash out perhaps try the trad heavy range, I have a couple of those too and they're still a beautiful bunch of cymbals, but they're obviously louder and will stand up to just about anything.
This is probably old news to you, but try
http://www.cymbalsonly.com and have a listen to different cymbals, the sound files up there are as accurate as you'll get without having the things there in front of you, and it can give you an idea of what you're after - maybe then you can go to a drum-store armed with a list of specific cymbals to try.
In short - lovely cymbals!