Can stratospheric high finance, which appears so tedious and complicated on paper, really be this much of a hoot? You can't imagine a more deliciously entertaining movie about the $25-billion leveraged buyout of R.J.R. Nabisco Co. The script by Larry Gelbart, based on a bestseller by Bryan Burrough and John Helyar, demonstrates that the former "MASH" head writer is as acutely clever and witty as ever. Former Nabisco president F. Ross Johnson (James Garner, pictured) delivers one-liners, double-entendres and sight gags in a wheeling-dealing Wall Street atmosphere polluted by greed, deception and tobacco smoke (HBO Saturday at 10:30 p.m.).