I ebayed a 1Gb sodimm for my laptop (Fujitsu Siemens V5535). Looking at the specs on the dimm it's faster than the one that's already in there (PC6400 v PC 5500 or somesuchlike) .
boot screen displays new ram OK
PC wont boot with just the new dimm in
linux distro (lubuntu) wont boot
Windows (Vista) very flaky (sometimes boots, drivers often crash)
memory tester that installs with ubuntu gives it the all clear.