There are lots of threads on this topic - do a search for stash and see what comes up.
Personally, when I see something I LOVE, I buy a 2-3 yards. If it's interesting, and think it's going to be used, I will sometimes buy up to two yards, sometimes just a fat quarter. If it's a neutral that will be used to blend, I'll get at least a yard. I definitely stock up when I see a decent price- black/white/gray/brown/red...assortments.
Also, if you are making a quilt, you can buy extra of the fabrics. I recently made a batik that required up to 90 different fabrics, so I stocked up on fat quarters. Didn't get that many, but made it to about 50 I believe. So now I have an awesome stash of batiks for other projects.
Remember this - if you HAVE TO HAVE the fabric, if you don't buy it, you'll never see it again. Also, if you don't get enough, you'll never see it again!
Another thing - there was a thread about different grades of the same fabric. Sometimes the manufacturers will make a lesser quality fabric for stores that carry less expensive fabrics (like WalMart) and a higher quality version of the same fabric for the quilting stores and other retailers like that. You may want to get a sample of the WalMart fabric and put it next to the higher priced fabric to see if it really is the same. Probably not from what I've learned on this very informative board!!
Good luck, and have fun!