Vendor memory specifications

When vendors lists a certain speed for a memory sku what is the likelyhood of clocking higher?

For example http://www.gskill.com/products.php?index=493 lists a Tested Speed of 1600MZ. Now I noticed when I run this memory in my stock setup it runs at 1333MZ. Okay, so the manufacturer is saying I can over clock it to 1600MZ. Is it possible, or may it be possible to overclock this to a higher speed or do I need to get a different skew rated for a higher MZ?

Thanks!

1333 = SPD , auto profile compatible with JEDEC standards
1600 = XMP , profile with settings guaranteed to work by G.Skill

Simply, you can set 1600 using XMP option in BIOS or just set it manually to 1600 CL9-9-9 1.50V.
Noone will guarantee overclocking above XMP profiles and it depends from used chips and some other things like chipset/memory controller. Some IC are overclocking better, some worse.

Thank. Makes sense. I'll be conservative and just use the XMP profiles.