Ys Great series to get into, even though the combat in the early Ys titles is pretty polarizing (personally, I like it!). The best ways to play Ys IMO are by picking up a Turbo Duo, and having a Windows PC with "AppLocale" and East Asian Languages installed.
English Ys releases:
Ys 1 (SMS)
Ys Book 1 and 2 (Turbo Duo)
Ys 3: Wanderers From Ys (SNES, Genesis, Duo)
Ys VI: The Ark of Napishtim (PS2, PSP)
For Ys Book 1 and 2, you can either go the Turbo Duo route, or pick up the Ys 1 and 2 Eternal game for PC - which features vastly improved graphics and you can actually swing your sword. The latter has a full translation out, too. I don't have the PC remakes yet, I am planning on picking them up soon though.
For Ys 3, the way to go is definitely the Turbo Duo. All 3 versions were translated, and the Duo version has janky scrolling, but Ys just ain't Ys without the amazing music, and the Duo has that amazing CD quality soundtrack.
Ys IV never came to the States, but there were 2 separate versions of the game that have completely different areas and stories: Mask of the Sun (SFC) and The Dawn of Ys (Duo)... I haven't played MOTS yet but The Dawn of Ys is supposed to be much better (I loved Dawn, it's probably my favorite of the series that I've played, and has great music and boss battles). Both games have full translations out for them, the Dawn translation expects you to own a copy of the original game (easy enough to find an import copy on Ebay, and not too expensive), you rip the original game to your PC, apply the translation patch, then burn the game to a CD-R... the Duo plays CD-Rs without a hitch, since it was one of the earliest CD systems, before copy protection became so necessary.
Ys V is SFC only, and a bit of a departure from the other Ys games. It supposedly has a translation forthcoming, but the game is reportedly difficult to hack. I haven't played this one yet.
Ys VI should be pretty easy to find on PS2, stay away from the PSP version, the loading is horrible. The PS2 game is pretty good, even if a bit unbalanced in some areas and requiring a bit of grinding... English voice acting is bad but you can put in codes to change it back to Japanese VA.
Ys: The Oath in Felghana has a translation coming, it's PC only. See below for the link to the translation site.
Ys: Origin is the most recent Ys game released, I'm willing to make a wager that RIGG is gonna translate this once Oath is released.
Stay away from the Ys PS2 remakes... I have Ys 3 PS2, it's got a lot of loading, and over much too quickly for it to be worth the price... Ys 4 and 5 PS2 are supposedly AWFUL, though.
http://hg101.classicgaming.gamespy.com/ys/ys.htm has a really in depth writeup of the series and all the different versions.
http://www.himeyashop.com has the Ys PC games available, though they are a bit pricey!
http://nick.serveblog.net/index.php has the Ys 1 and 2 Eternal, and Ys IV: The Dawn of Ys translations. These guys are working on the Oath in Felghana translation which should be released soon. Mejilan from NeoGAF is a beta tester for it. I might donate so I can get a copy, since I recently picked up the game.
If you have any other questions, feel free to ask me! Always willing to help, as I only got into this series in '05 (after reading about it and salivating for years). Bought a Duo mostly for Ys 1-4
