Buying a new imac, which memory should i get? For the new imacs that just came out, which memory upgrade should I get?

This:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820226095

or:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231428

The apple site shows that it's using 4GB 1333MHz DDR3 SDRAM - 2x2GB ram. Would the 1600 ram work as an upgrade or would it be incompatible? And what's the speed difference between the two, is it noticeable?

Thanks.
Those are desktop DIMMs, you want the laptop DIMMs.
Try this search instead. or this search which starts a the Mac search below.
1600 would work, but it wouldn't be
much faster as they would run at the lower frequency, but might have better latency.