Stick with CS3 (if you have it already).
From what I'm reading about the two, there is nothing ground-breaking about CS4. You're much better off spending much less on CS3 if you won't be utilizing the new features included with CS4.
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All applications in CS4 feature the same user interface, with a new tabbed interface for working with concurrently running Adobe CS4 programs where multiple applications can be opened inside multiple tabs contained in a single window.
[3] It was announced that nVidia
CUDA technology is now supported to allow for the accelerated encoding of video. Adobe CS4 is also developed to perform better under 64-bit and multi-core processors. Adobe Photoshop CS4, Adobe After Effects CS4, Adobe Premiere Pro CS4, and Adobe Soundbooth CS4 will officially support
64-bit. However, Adobe CS4 64-bit will not be available for
Mac OS X.
[4] In early testing of 64-bit support in Adobe Photoshop CS4, overall performance gains ranged from 8% to 12%. Those who work with extremely large files may realize noticeably greater gains in performance, in some cases as dramatic as ten times the previous speed. This is because 64-bit applications can address larger amounts of memory and thus result in less file swapping — one of the biggest factors that can affect data processing speed.
So if you can live with swapping between windows/programs, increased speed with proprietary NVIDIA technology (in which
you would have to have a supporting GPU from NVIDIA), increased performance with 64-bit support (which you need a 64-bit system to take advantage of), and increased performance with multi-cores (which anyone can benefit from). If you don't have the hardware to begin with, though, I wouldn't bother forking over much more for a marginal 8-12% in efficiency.
There are new features with the programs themselves, but only a professional would actually be looking at what CS4 has to bring to the table. CS4 doesn't put too much to consider the upgrade, unless you absolutely had to have the latest and greatest. Just at a glance:
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Photoshop CS4 features additions such as the ability to paint directly on 3D models, wrap 2D images around 3D shapes, convert gradient maps to 3D objects, add depth to layers and text, get print-quality output with the new ray-tracing rendering engine, and enjoy exporting to supported common 3D formats; the new Adjustment and Mask Panels; Content-aware scaling (also known as
seam carving[8]); Fluid Canvas Rotation and File display options.
[9] On 30 April, Adobe released Photoshop CS4 Extended, which includes all the same features of Adobe Photoshop CS3 with the addition of capabilities for scientific imaging, 3D, and high end film and video users. The successor to Photoshop CS3,
Photoshop CS4, is the first
64-bit Photoshop on consumer computers.
[10]
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[Illustrator] CS4 was released in October 2008. It features a variety of improvements to old tools along with the introduction of a few brand new tools. The ability to create Multiple Artboards is one of CS4’s main additions. The Artboards allow you to create multiple versions of a piece of work within a single document. Other tools include the Blob Brush, which allows you to make a more natural paint brush stroke (reminiscent of Microsoft paint or other non-vector drawing programs) which then becomes vectorized, and a new gradient tool that allows for more in depth manipulation of colors.
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Adobe Flash CS4 Professional (as version 10, released on October 15, 2008):-- the new version of flash, code named, "Diesel" — not to be confused with the latest public prerelease of the Flash 10 Player, code named "Astro" — contains
inverse kinematics (bones), basic
3D object manipulation, object-based animation, an enhanced text engine, and further expansions to ActionScript 3.0.
I would look into the programs as to what they offer with the upgrade and determine whether CS4 is actually right for you or not. As of the moment, I would wait until we see what CS5 brings to the table, sticking to CS3 in the meantime (especially if you're an amateur/intermediate on a budget).