You're wasting your money if you go over 8GB. Heck, for your intended use, you're only going to see marginal returns on your investment going over 4GB.
For a computer for MMOs/LoL I'd prioritize components as follows: i5-2500K (or equivalent Ivy bridge) > 8 GB > Graphic Card > SSD. Meaning, once you hit the spec in the list above, spend the budget making sure you hit the spec in the next level. For graphics cards/SSDs, you'll probably see way more return on your investment spending a lot there vs spending a lot on tons of RAM.