ATI X1900 Series Overclocking Guide
 

You may think you are experienced, or have overclocked before and therefore may think this guide is of little use to you, but I would have to say that you are incorrect. This guide is for both the experienced and inexperienced overclockers. Some parts may be boring for those experienced in the art and they may skip to the highlights and key points of performing this task.

http://www.madshrimps.be/gotohowto.php?howtoID=61

After you overclock with with ATI Tool are you still able to play a video?

I've tried using the method (with different voltages and settings, of course) on my X1800XT but when I push the core above 640, video locks up--whether using WMP or MP Classic.

None of the reviews I've seen ever mention playing video after all the overclocking. There's an open debate on this over in the Rage3D forums.

just a warning to always keep an eye on your GPU core temps while overclocking, if it gets too hot, you might cause permenant damage http://forums.techpowerup.com/showthread.php?t=14098

I wrote to ATI about some Crossfire difficulties I was having, and they suggested that the problem was I had dual 12V rails. They told me that I needed to have a single 12V rail with over 25A. I have a total of 36A over the dual 12V, so would I have to combine the rails, or would a single work? Also, I can't find a splitter for the PCI-E Power cord, an 8-Pin Splitter so I CAN use a single rail. Any suggestions?

I needed to upgrade a 550watt high power supply to get these cards to work overclocked.
I also had 36Amps ( I think) on the 12 volt rail, but it didn't work.
A Zippy 700 did the trick.

upgrade your PSU.

I've got an Enermax Liberty 620W. It should be working fine with Dual 12V/22A for a total of 432W/36A

yonah setup with Crossfire needs 500watt to opererate in non overclocked state, has been tested here.

its a 620 Watt PSU minimum power, with a max of 700W. I meant that exclusively over the 12V Rails there is 432 Watts availible. Here's the stats:
http://www.enermax.com.tw/english/pr...ay1.asp?PrID=5

single 12v maxxed out at 22A, simply not enough :(

Is there a way to combine the rails? Or do I have to get a new PSU?