Second terms are all about legacy building, and plans being drawn up by the White House would make tax and retirement reform the hallmark of another four years for Dubya, insiders say. "These are big ideas, Reagan-sized ideas," says a key adviser. Already, treasury and congressional Republican leaders are considering a total rewrite of the increasingly complicated tax code, possibly junking it in favor of a European-style value-added tax or a flat tax, say officials. It's part of a larger administration effort to find a replacement for the 1970s-era alternative minimum tax that hits about half of all Americans with added taxes. "That study will be the vehicle for bigger changes," says an administration official. Part 2: President Bush also plans a major push to let Americans put a tiny portion of their Social Security tax into personal stock accounts. But insiders say don't look for Bush to provide any pre-election details. Instead, says one aide, the prez will use his Republican National Convention speech this summer to "paint his ideas in broad strokes."