I have increased the ram to 4GB. I run vista sp1. Wakeup from sleepmode doesn't work anymore. The cpu fan turns at highest speed with black screen. Power out and restart will resume vista normally as coming out of hibernation. The sleep/wakeup procedure works fine if only 2GB is applied.
What could be the cause of this malfunctioning?

Not that this matters much any more, but, I have never found hibernation to work properly, so I have always disabled it. And with it costing $3 a month, I dont see a point in worrying about it.
Btw, just so others who may not know, running 4 gigs of ram on a 32 bit system will only make use of 3 gigs. But, I won't recommend 64 bit version of vista. Too many app conflicts. Until manufacturers both (soft and hardware) get on board there will be more issues than its worth.
At least thats my opinion, from what i experienced.

Have you checked that the personal or laptop computer can take 4GB of memory? The other option could be that you have the wrong type of memory for your computer. You could use a online scanner such as the one from Crucial to see what your computer needs - Memory upgrades from Crucial.com - About Crucial:Media Toolbox
Matthew
HP