We use QSM SLIM-Estimate tool. Its works well for the planning stage. Basically, you enter parameters such as use case count, complexity, productivity index etc and SLIM generates the number of total man hours required for the project as well as the resource count and break down of hours by RUP phases.
For change requests, you can probably rerun the SLIM and compare the deltas.
SLIM Estimate [ June 06, 2004: Message edited by: adnaan sikandar ]