You should take malkore's advice. I thought the same thing, buy aragamax sand, then buy live sand to "seed" it. But if you do get lots of live rock you can save a lot of $$ buy sticking to the non-live sand.
I would suggest "borrowing" a few cups or more of live sand from a trusted/established tank to seed your non-live sand. Some of the live sand that you buy pre-packaged only contains the beneficial bacteria and none of the beneficial worms and such. My LFS
is gonna give me a few cups from their tank.
And one last thing. If you do decide to buy the live sand to seed the aragamax sand, I don't think it really is an issue if it migrates to the bottom. First off, if you put the live sand as the top layer, I don't think it will find it's way to the bottom for at least a few weeks or even a month or so. By then, most of your non-live sand will become live sand. If you are worried about aesthetics, I don't think you'll be able to distinguish which sand is which or even be able to distinguish a barrier between the to different sizes.
Although I am a begginer, I have done months of planning and asking questions. Experts please feel free to critique. I'm just passing along some advice that others have told me.