Foam cushioning should be okay... did you order it off the net, or in PC World, as the cushioning on OEM hard drives should be good enough to use the base and surround cushioning.
But don't use the top cushioning as it needs somewhere for the heat to go, just make sure its as secure as possible on the cushioning.
Seagate do make 2.5" drives with shock resistance of upto 1m by using head locks and platter springs etc, however they are only available in external Freecom Toughdrives atm.
Provided you're ride isnt too rough, and you take care when going over speed humps and bumpy roads, I think you should be okay with basic cusioning.
You probably only have a matter of a year or so to wait before SSD hard drives start coming down in price considerably and I have seen 128GB/256GB drives touted in the press lately and of course 32 and 64GB are already on the market, so you're problems will then be over.