For years collectors and the news media have been speculating about when prices would finally top out. Spring sales estimates don’t suggest pessimism. The auction houses clearly hope that things will play out as they did three months ago in London, when, despite global economic queasiness, a Francis Bacon triptych sold for $51.6 million. Now two Bacon triptychs, whose owners no doubt want to capitalize on that high, are going on the block, at estimates of $25 million to $35 million (Christie’s, shown above) and a whopping $70 million (Sotheby’s).