The delicatessen gone berserk. An Upper West Side institution, the selection at Zabar's is wide enough to cause a panic attack, as is the size of the weekend crowds. You never knew that there were so many cheeses in the world, or that smoked fish came in so many varieties.      While you're there, go to the bakery, buy a picnic basket, pick up some caviar...but don't forget what is perhaps the best part of Zabar's -- its second floor, where you'll find probably the best bargains on one of the best selections of kitchenware in New York.      Sign of the times/possible urban myth: shoppers riding their limos to the door to make the Sunday morning opening bell. next: Starbucks