Which Sample Rate & Buffer Size should be used

Depends on your sound card.  I'm using a mixer with built-in 24 bit card that runs at 48,000 sample rate.  So that's what I use.

@ Fuse Promote -> I am using the Behringer DDM 4000 & had a look at the tech specs on it. It has a 32 bit card but doesn't indicate what sample rate.

What would you recommend I use.

Also, with the change of sample rate, does this have to be done when tracks are imported to Rekordbox or does it come into play when the library is export out of Rekordbox?

The sample rate setting in record box only affects the audio as it plays inside the software. It does not affect the audio files themselves, or how they are played on the CDJ players.

As for buffer size, you want to use the lowest setting that does not cause clicks or pops in the audio when it plays. If you hears glitches, then raise the buffer until they go away.

Higher quality = better sounds = requires more resources = higher latency on an older computer (if you have a newer computer, no problem)

@ Pulse -> please forgive me for my lack in knowledge of computer techno jargon...coz I really follow. Excuse my ignorance.

I am using a new computer (2011 with windows 7 enterprise)

@Fuse Promote -> so you recommend the buffer size to be 64 samples.

Now, I already have over 5000 tracks on my rekordbox library at the moment, which were analyzed with the factory settings (so to speak). ..... Will this mean to go down to 64 samples, I will have to redo everything.

Or will it change when I am exporting to USB/ external hard drive?

I use the default 512 samples.

No, the sample rate does not affect your music.  It only affects how the Rekordbox software plays audio on your sound card.  It has nothing to do with your audio files, exporting, or LINK to CDJ players.  It is only for listening to audio inside rekordbox.

I'm having trouble with this on the RX2. Have increased the buffer to maximum and I'm still getting crackling and distortion. Can anyone help?

@Lrobson76 > The buffer setting depends entirely on your computer - what's the spec of the CPU / RAM?

I'm running Windows 10 Home edition on an Acer Aspire V 15 laptop. It has an Intel core i7 CUP @ 2GHz with turbo boost up to 3.1GHz.
RAM is 8.00GB
64-bit

That is meant to say CPU, not CUP. Predictive text for you lol

 I have macbook 2012 4 gb ram i5 cpu > controller ddj-rb 
what would be the best buffer and sample rate for this? Thanks! 

@Yas > Every computer is different; two DJs using identical computers may require different values simply due to any other background applications / services running on the computer. My recommendation is to continue to adjust the settings as low as possible until you get hiccups / stuttering / static / other undesirable effects, then go up one or two notches.