I am about to get my new custom gaming PC and play ArmA2 for the first time... been waiting for a year.
I have been playing OFP and ArmA 1 on a daily basis from the first day they came out and I know that the game can be improved massively by mods etc.
I am going to play the storylines first.

What sort of mods can you guys recommend for me to get before I play ArmA 2 for the first time.
I always enjoyed the sound and smoke mods in ofp/arma...

JTD fire and smoke....
zeusAI
CBA addons (I think their needed for most mods)
HIFI sound mod
31st mod is great if you like WWII fighting...

those are just a few I use.

I believe that having both ArmA2 and ArmA2 : Operation Arrowhead is enough to get a nice experience of the game ... you can add some salt with the BAF DLC ...
But let's play the vanilla BIS work before adding anything ...

From my point of view, I will just add :
- Rug HD from Wolfrug : this small addon is adding the high dispersion weapons we enjoyed with OFP JAM weapons.
- RKSL contermeasures from RKSL Studios : this small pack is needed to add contermeasures an flares to all BIS made planes.

http://forums.bistudio.com/showthread.php?t=96591

This is a good thread to consider when picking mods.