Minor point:
Quote -"XP 32-bit only sees about 3.5GB at the most, and windows 7 32bit only sees 4GB" End quote
Win XP 32 Bit and Win 7 Have Identical memory limitations. That is Both will "see" 4 gigs and Both will have between 3.0 and 3.5 as Usable. 4 Gigs (limitation for 4 gigs - memory addresses that would push above 4 gigs such as GPU memory, Dos (Yes ther is a small dos address), ect.
Windows 7 just made a cosmetic change in how 4 gigs is reported - TO elieminate users asking that question!!
As To speed:
(1) Very little performance diff between 1333 and 1600.
(2) Ram will initially be set to the Default spec. IE My DDR3-1600 in my SB system will come up as 1333 and to get the rated 1600 must change to it. IE In bios select XMP, Use profile 1.
... A. For a SB system the bclk is set to 100 MHz, changing the ram from 1333 -> 1600 does NOT change the BClk therefore does NOT effect CPU speed
... B. For My I5-750. When changing the ram from 1333 -> 1600. When selecting XMP, profile 1, IT raised the Blck Freq (and memory Multiplier Ratio. This would have raised the CPU Freq (OC); HOWEVER it also lowered the CPU multiplier so that the CPU ran at it's default value.
.. Above is to explain, each MB may have slight variance of How it is implemented.
Bottom Line, if 1333 and 1600 are nearly the same price (Also CL ratings are the same) go for the 1600.
Also FWIW - I have 8 gigs Ripjaw in I5-750 and 16 Gigs in I5-2500k - All the same modules, DDR3-1600 CL7 @ 1.6 V. Not one problem and they worked well right off the Bat. NOTE: Due to SB, the recommended DIMM Voltage is 1.5 Volt (Max Intel spec is 1.575V).