If you only do light multi-tasking while gaming, 8GB should serve you well for a fair while.
Personally, I would simply get 16GB and not have to worry about RAM for the foreseeable future. Why? Because DDR4 should enter mass-production next year and that means a drastic reduction in DDR3 supply and corresponding increase on prices so getting 8GB now + upgrading to 16GB in 1-2 years will almost certainly be far more expensive than getting 16GB today. DDR3 prices have already gone up 30-50% compared to a year ago.
Buying what you need now and upgrading if you need more later is fine early in a DRAM generation where production is still ramping up and costs are going down. At the end of a DRAM product cycle like DDR3's though, prices are going up as DRAM manufacturers shift production to newer memory types so upgrading RAM on an existing system later can be a very expensive option - sometimes to the point of not making any economic sense.
Ex.: last year, I was originally thinking of upgrading my C2D to 16GB RAM but changed my mind when I saw that 16GB DDR2 cost $330 while a new i5+h77+16GB DDR3 only cost $380. It made a lot more sense to pay $50 extra for the whole platform upgrade.