HSA Improvement and Expansion Act of 2007 - Amends Internal Revenue Code provisions relating to health savings accounts (HSAs) to: (1) allow HSAs to incorporate flexible spending and health reimbursement arrangements; (2) increase the annual HSA contribution limitation; (3) permit the use of HSAs to purchase health insurance; (4) allow the payment of certain medical expenses incurred before the establishment of an HSA; (5) allow veterans eligible for service-connected disability benefits to establish an HSA; and (6) allow spouses to make increased catch-up contributions to a single HSA.