What Bios version are you using? Latest one appears to be F.0D N (4 Nov 2008).If your using something else - get it here:
http://h20000.www2.hp.com/bizsupport...tem=ob-66972-1
From what I have seen - your not the only person to run into this issue. The board specs show it will handle 4 Gigs
http://h20000.www2.hp.com/bizsupport...dTypeId=321957
The SoDimms must be DDR2 at 667mhz. It is possible that if the speed is off, the laptop may just run one fine (kicking it down to the lower speed) but with both sticks it chokes. Either stick alone it boots fine?