How do you store a whole lotta LP's?

As mentioned, modular furniture can provide a prety cost-effective and reasonable looking solution if your not prepared to go to custom built shelving. IKEA (http://www.ikea.com) offers some solutions that some have found work well. Look at the Bonde, Billy and Ivar series.

With the Bonde series, the trick is to purchase a combination of the 14x86 and 28x86 bookcases. These have shelves in 13" widths. They come in wood veneer, or white for a lesser price. See:
http://www.ikea-usa.com/product_presentation/show.asp?productnumber=00030064&type=III
http://www.ikea-usa.com/product_presentation/show.asp?productnumber=00030063&type=III

With the Ivar series, the trick is to buy separate 13" (approx.) shelves and 6' uprights. You connect them to form two columns of shelves. With each shelf at 13" wide, you don't have any sagging and you get good vertical support for your LPs. The six foot uprights easily allow for 4 shelves; to get 5 shelves you need to start with the first shelf on the floor. (The Ivar series is sold on a completely modular basis - you order it stick-by-stick to match your needs.) See:
http://www.ikea-usa.com/product_presentation/show.asp?productnumber=00005050&type=III

At one time, Ikea sold a line of modular 13" x 13" shelving that was a perfect inexpensive solution: the Expedit series. It never appeared in their website, but was available in the stores. I don't know if it might still be available.

With the Ikea Ivar units, there is no backing to keep LPs from being pushed past the back end of the shelf. You have to either live with that issue and be careful in re-shelving, or you can mount a rail across the back side of the uprights about 5-6" above the top of the shelf.

I use some modular shelving (older, so not the Ivar) that has both 10 3/4" deep shelf units and 17" deep shelf units. Both work fine for storage. What I find is that the 17" shelves are nice for reducing the amount of dust that can accumulate on top of the LPs because they are further recessed. (I'm storing several thousand LPs, so rotation takes a while.) Also, there is space to the front of the 17" shelf to rest something or to set an LP face-forward temporarily pending re-shelving. The downside is that the lower shelves can be a bit difficult for viewing the LP spines due to that same recess. This would be exacerbated with 20" deep shelves.