have you thought about what will happen when your visitor is using a popup stopper? these things stop javascript open window functions.
ergo your people will be pissed and bugger off because they don't want to have to turn them on and off all the time.
i assume that you are doing this so the people registering at your site can check the details b4 they click the final ok button and if this is the case then why not use 1 page to validate the page, display any info sent to it etc, or just send it a validating page to check the info given and if this is ok then you can send them on to the final check info page without opening a new one.
if u absolutely, and i mean that entirely, have to open a new page why not use the old but very rarely used (ok so you can't limit the size of the window or remove menus and such like) target="blank"