They're releasing Rune Factory: A Fantasy RPG for the DS on the same day P3 comes out, it's a Hack and Slash game with Harvest Moon's farming and marriage system. A lot of HM fans say it's really good.
Also a sequel to the main series (BTN, AWL, etc) is coming out in February, with new characters and is set on an island.
http://www.natsume.com/graphics/e32006/E307.pdf I'm a fan of the HM series as well, and I'm going to get a DS lite in the future to play those games as well.
Ys has terrible loading times on the PSP, if you get it I highly recommend playing it from the memory stick.