I would put off Awakening if you plan on playing older games. I mean if you want a physical copy buy it right now, but it will be hard to go back after Awakening. You might feel limited.
I'm not sure if I would or not. I mean I do tend to do that if I get into a series, I'll go back to the older games...but I have yet to be sure I'll enjoy this or not. It gets so much praise, I feel like I gotta give it a shot.
I already got a copy of Radiant Dawn, but I have heard what people have said in the past before about how it is the closest thing to a direct sequel for Path of Radiance, but I've looked that one up, and it's so expensive to find a copy now, I dunno when I'd have a chance to play it.