The card you refer to has max TDP of 150W and there are posts on it drawing even more than that, supposedly fixed by new drivers.
Take a look at the latest Z400 QuickSpecs and see if there are any HP approved cards drawing that much power. I don't think so. Then, you need to think about getting one of the higher capacity Z400 power supplies, either HP's version or home made (I'd go for the HP version myself). You can find that via Google. Nothing that money can't fix.
Finally, there are lots of mods in your link above.... which do you refer to? Remember if you put in an aftermarket PWM controlled fan you'll get its rated RPMs and then also the influence of HP's stock motherboard PWM throttle, which can make a nice quiet fan go way too slow in practice. The HP engineers have worked hard to get balance between the CPU heatsink fan they chose and the motherboard's PWM control software for that heatsink. You can increase all the fan speeds from BIOS, but not just one.
I'm running the stock "Performance" heatsink/fan in my Z400 which has a X5690 130W max TDP processor. You have seen its W7Pro64 Windows Experience Index scores above, and below are the processor temps. Personally I'm sticking with the stock HP Performance heatsink/fan because I can hardly hear any of the fans in this system.
