The LiIon batteries are backward compatible with the older tools.
A couple mfg's have made the batts backward compatible (e.g., Dewalt).
If you buy an old tool or tool body and then buy the new LiIon batteries and charger (you'll need the new charger) then y'll likely spend as much as if you bought the new tool WITH the batts.
Ryobi and others are doing this to support their customers who have older tools already. It makes great sense to me, and if I were a Ryobi tool owner, I'd be thrilled.
Makita, on the other hand, did NOT do this bkz they timed their LiIon upgrade with a total redesigns that make the new batts incompat with the older tools.
Not to knock Ryobi, but if I were you I'd get the Dewalt instead. L___WS is having a killer sale. For $99 you can get an 18v that'll be forward compatible with the Nano batts when they come out. Now THAT's a killer deal