If you have to rescale images, and want high resolution output, you might try this PS CS and CS2 only plug-in, Genuine Fractals 4.1. It is not free, but is on sale for a short time for $79.95, a 50 % savings. http://www.pluginz.com/download/13019?category=0 .

If you buy now, you get a free upgrade to version 5 compatible with CS3, as soon as it is released, which normally sells for $159.95, the upgrade is $69.95.  The home site does not have this sale on, but you can get additional info here; http://www.ononesoftware.com/#SlideFrame_1 GF has demos available. 

I have used Kneson Imagener Pro for years and GF totally blows Imagener away at an 800 percent rescale. 

I see that it is no longer on sale.

Yes, the sale is now over because onOne Software have released the Genuine Fractals 5 Beta, a free upgrade to 4.1 for anyone who wants to try the beta and help test. 

If you bought GF 4.1 software after December 18, 2006 you can get the final release of GF 5 as a downloadable upgrade to 4.1. GF 5 will be compatible with PS CS3 and the Win XP and the new Vista operating system.

An Additional note; I recently received e-mail that Adobe will not make CS2 fully compatible with Vista. If you want full Vista compatibility you will have to upgrade to PS CS3.

I am currently running PS CS2 on Vista Basic 32 bit and have some minor issues. 

The Register your Software screen boots each time you open PS. This action can be turned off by changing the compatibility settings of the PS exe file to Run as an Administrator. 

The UAC will block opening of PS each time you try to launch. You must choose the Allow Software to launch to get PS CS2 to open. Otherwise, everything else works.   You could turn off UAC, but that is not recommended.

Adobe has also put out info about the Auto Updater, and the possibility of a Serious Security Vulnerability. You must upgrade the Auto Updater or Hackers can gain control of PS and your operating system. 

I've also had excellent results scaling images with Alien Skin's Blowup.