I think that is SO important to have many items in your shop. It really increases the liklihood of getting found when people are searching.
I definately think that you should have some "ready to ship" items available (your most popular items, of course). When people want something, they want it now! Of course, many are willing to wait for a special item. But I see many last minute shoppers....(that baby is due any day now, oh, Christmas is only 1 week away, etc). But having things ready, you can take advantage of those types of customers.
All the best!