Congrats on having the room for your collection. I think that problem plagues many collectors here. I do have some recommendations for you to consider.
First is the glass shelves you see alot in collections. They are from IKEA and are about $60.00USD apiece. They have four shelves that are not adjustable but all glass! Which is nice for viewing and they are quite stable. The shelves themselves can hold several decent sized figures and they keep the dust off your more valuable figures:
Second is an "open air" styled shelf that IS adjustable. The strut system can be nearly seamlessly combined with more to create longer shelves and stacking. They are a MUCH cheaper alternative and are available at Wal-Mart (in the bathroom department actually). They come in white and wood for a very reasonable $12.00USD: