CS3 is fully compatible with Intel Macs. CS3 is faster in general. CS3 looks better. CS3 has many new features such as smart filters.

(I work for adobe so here it goes, keeping in mind this is in no way an endorsement, only the facts on new features / etc.) Smart Filters Add, adjust, and remove filters from an image without having to resave the image or start over to preserve quality. Nondestructive Smart Filters allow you to visualize changes without altering original pixel data. Advanced compositing Create more accurate composites by automatically aligning multiple Photoshop layers or images based on similar content. The Auto-align Layers command quickly analyzes details and moves, rotates, or warps layers to align them perfectly, and the Auto-blend Layers command blends the color and shading to create a smooth, editable result. Streamlined interface and palette management Maximize screen space for editing while keeping essential tools accessible. Palettes are now arranged in convenient, self-adjusting docks that can be widened to full size or narrowed to icons or even a thin, self-revealing strip at the edge of your monitor. Better raw image processing Process raw images with increased speed and superior conversion quality using the Adobe Photoshop Camera Raw plug-in, which now adds support for JPEG and TIFF formats; new tools including Fill Light and Dust Busting; compatibility with Adobe Photoshop Lightroom™ software; and support for over 150 camera models. Enhanced Vanishing Point Take perspective-based editing to a new level with the enhanced Vanishing Point, which lets you create multiple planes in an image, connect them at any angle, and then wrap graphics, text, and images around them to create packaging mock-ups and more. Enhanced 32-bit high dynamic range (HDR) support Create and edit 32-bit images and combine multiple exposures into a single, 32-bit image that preserves the full range of a scene — from the deepest shadows to the brightest highlights. New image processing and alignment algorithms deliver superior results. Black-and-white conversion Easily convert color images to rich black-and-white and adjust tonal values and tints with a new tool. Experiment with the included black-and-white presets, or create and save your own custom presets to achieve the best results. (http://www.adobe.com/products/photoshop/photoshop/upgrade/)

nothing important.just a few hundred dollars.