It doesn't matter too much and the rest of your system specs. will determine what RAM you can use.
It looks like you're talking about DDR3 so nearly everyone buys 8GB of 1600MHz these days as you can pick this up for around £40-£50.
If you want more specific advice please list your processor, motherboard, the CPU cooler you're using and what RAM you have now.
I assume you have a 64 bit operating system so you can use more than 4GB?
1600MHz is enough.
Faster RAM offers little benefit with Sandy Bridge.
Have a read:
The Best Memory for Sandy Bridge
ok and is it not better to buy timings 7-7-7-7 or 8-8-8-8 than 9-9-9-9 etc....
Read the article.
Faster RAM and/or lower timings only make a very small difference.
90%+ of Sandy Brdge systems on this forum with a "K" series processor will use 1600MHz C9.
1600MHz is enough.
Faster RAM offers little benefit with Sandy Bridge.
Have a read:
The Best Memory for Sandy Bridge
if I buy 1333 will i be losing a lot in performance or not?
if I buy 1333 will i be losing a lot in performance or not?
I would also like to know this. Loads of friends cringe when I put a 1333 in my mockup build with a 2500k sandybridge i5.
(I am a noob when it comes to PCs)