First, let's talk about the decode acceleration performance of the competitors. HD playback on the PC has come a very long way in the last year, with the Radeon 2600 XT Compare Prices on Radeon 2600 XT Video Cards and GeForce 8600 GTS Compare Prices on GeForce 8600 GTS Video Cards series cards delivering on their promise to make HD 1920x1080 video playback a reality on slower CPUs.
While the low-end 2400 PRO and 8400 GS show a lot of promise in the decoding arena by offloading CPU work almost identically to their bigger brethren, show stopping issues we experienced - like the maximum resolution cap of the 2400 PRO and the flashing artifacts on the 8400 GS - make them difficult to recommend at this time.
Having said that, if you have already purchased a 2400 PRO or 8400 GS for video playback, I wouldn't gnash your teeth just yet. It looks like driver fixes are well on the way for the 2400 PRO, and I'll be surprised if the 8400 GS doesn't also get the same treatment soon.
As far as quality enhancement post-processing like noise reduction and the removal of jaggies, I'm not sure the 2400 PRO and 8400 GS will ever have the processing power it takes to enable these features. They are currently disabled in the drivers, and therefore cannot be tested.
The higher-end 2600 XT performed very impressively in both Windows XP and Windows Vista, walking away with top honors in our tests. The 8600 GTS offered us a mixed bag of tricks, showing us no post processing in Windows XP, but doing a really good job in Vista - with the exception of losing a few points in the noise reduction category.
In the final analysis of this round, we must note the 2600 XT's low street price compared to the 8600 GTS, HDMI output with integrated audio controller, and post-processing enhancements on both Windows XP and Vista. It's hard not to recommend this card for a dedicated home theater PC. The 2600 XT's only drawback is its limited gaming prowess; as a rule, the card is restricted to 1280x1024 for good performance in newer titles.
If gaming is higher on your list of priorities, but HD video is still important to you and you don't care about HDMI, the 8600 GTS might be a slightly better fit - on the condition that you're using Windows Vista, as the 8600 GTS doesn't perform any post-processing in Windows XP at this time.
If gaming is your priority, you might want to wait and see if the upcoming 8800 GT and 3x00 series Radeons can up the ante in the gaming/HD video arena...