SURPLUS ESTIMATE HITS $5.6 TRILLION
Projections of federal budget surplus are revised upward once again, to $5.610 trillion over the next decade, prompting renewed debate between Bush administration and Democrats over whether government can afford everything Pres George W Bush promised during campaign in terms of tax cuts and new spending; $2.488 trillion of total projected surplus would come from Social Security system and by agreement of the two parties will be off limits; Democrats concede that bigger surplus estimate will make it easier for Bush to sell his tax cut plan, but insist that size of tax cut should be determined by how much money is available after budgeting for other needs (M)