I bought 32GB DDR3 for my XPS 17 and it arrived today. It's only £78 delivered for 16GB (2 x 8GB) so if you are still looking for 16GB it'll be worthwhile having a look around for it.
One problem though is that my XPS 17 OS is Win7 Home Premium which only allows using 16GB of the 32GB lol.
I need to upgrade to Win7 Pro or Ultimate Anytime to make use of the extra 16GB but I might just do a fresh install of Windows 7 Pro OEM I bought a while ago for a different PC but didn't use and keep the anytime upgrade money for Windows 8 Pro which hopefully shouldn't take too long to be released.