New RAM doesn't work on XPS M1710 laptop?

Got a Dell XPS M1710 laptop which originally came with 2x1GB (PC2-5300 DDR2 SoDIMM), no free slots remaining. I wanted to upgrade to 4GB, so I got a set of Kingston HyperX KHX6400S2ULK2/4G (2x2GB) PC2-6400 SoDIMMs. According to Dell Tech Support, this RAM (or actually, any regular DDR2 SoDIMM of at least 667MHz up to 2GB per piece) would be supported by my laptop without problems.

However, after I replaced the RAM and turn on my laptop, I only get a lit numlock and flashing capslock and scroll lock indicators, and it doesn't do anything else. :emotion-9: 

Got on the phone with Dell again and they informed me the lights indicate a problem with the RAM. However I tried the new RAM in other hardware as well: tested it on 2 different laptops, ran memtest86 on one of them, no problem whatsoever. I also tried them each separately in my own laptop (using only one at a time, tried each slot seperately) but the problem remains.

Does anyone have a clue what can be wrong here? And if there's any possible workaround? (guess not, but maybe some magic bios setting or whatever..?)