Desktop freezes when overclocking my ATI GFX card...

Here's the deal. I'm using the HIS X1950Pro 256mb. This is the power supply that I'm using:

http://www.ebuyer.com/UK/product/124926

I've just downloaded and installed ATI Tray Tools. My default card clocks are 580mhz (core). I'm not going to overclock the memory, since I'm only using the resolution 1440 x 900. I set the fan speed to 100% then overclocked the core to 650mhz. After using the 3D renderer, temperatures started to increase from 39'c (idle) to around 45 - 48'c. After about 10 seconds, everything freezes, then crashes. My screen just turns blank, yet the PC seems to be still essentially running. I can hear the fan reduce it's noise level, meaning that it's slowed it's speed down. Then I just have to restart my computer, and everything is fine - the card has reduced it's clock speed back to defaults, and temperatures are 39'c. I've tried doing this twice, and it does the same thing both times. Note that when I overclock the core to 620mhz, everything is fine. I believe (looking at the box) that this is the "turbo" mode for the card. Well that's it. As usual, any help or suggestions are welcome, thanks!

Oh **** I'm such a moron. Thanks very much mate. So I need ATI Tools? Not ATI Tray Tools that I'm using now?ruuuj

Uh-oh, check this out.

http://www.techpowerup.com/atitool/

Scroll down until you get to "Changes in this version." It says that 'Only X1950 Pro overclocking is not supported.' :( What's with that?

Let me just warn you now : X1950Pros do NOT OC well.

Most wont do anything at all... I have one and cant get far on the core, but even 1mhz on the memory freezes the PC!