What version of Photoshop are you using? In the version of Bridge that came bundled with CS3, there is a function that does just this and it has worked for me flawlessly so far.
In Bridge, go to Tools > Photoshop > Image Processor... and Photoshop will open if it isn't already. The resulting dialog box gives you various options for saving the converted images into different formats, quality levels, and sizes, where they will be output, and you can even run other actions on the image batch at the same time. For me, it's been a wonderful tool.
Scott
Also a self-proclaimed artist