It's all going to depend on what CPU you have. Intel's Sandybridge 2XXX series only supports 1333 Mhz RAM officially, but runs 4 DIMMs at 1600Mhz without issue. The Sandybridge 3XXX and 2XXXm series (enthusiast and mobile platforms) both officially support 1600Mhz but at only 1 DIMM per channel, however running 2 DIMMs is just fine. AMD's bulldozer platform supports 1866 Mhz RAM out of the box. Nothing officially supports 2133Mhz RAM.
If you're going with an Intel solution, I highly recommend getting 4x4GB 1600Mhz RAM, this will run just fine at the advertised speeds and timings despite not being officially supported. This means that while it will require an overclock (just select the XMP profile in BIOS otherwise it will only run at 1333), it should be stable and effortless in 99% of all cases.
EDIT: 4x4 @ 1600Mhz will only run you around 100-120 bucks (less if it's on sale), probably far less than 1866 or 2133.
and should I max out all slots now? i was thinking more like 8Gb today, + 8GB later (nxt to nxt year I think).
& since I am not much acquainted with installing dual channel RAMs in preferable slots, i wud also like to know in what config. should I place my RAM? 1st & 2nd slot or 1st & 3rd slot?
its gonna be probably a 2X4GB 1600 MHz now.
first and third slot is where you should install 2 sticks of dual channel memory (or 2nd and 4th slots).
I can't remember where I read it, but there's an indepth article comparing memory speeds, and basically what the conclusion said was you won't notice a big difference past 1600mhz. Go for 1600 and put any extra money towads larger quantity, such as 16gb vs. 8gb. With 4x 4gb sticks you would then fill all those slots up