Just to make sure, do you need more RAM?
If you don't know, you should check in Task Manager and see if you're using your 8 GB now.
If your RAM usage is regularly getting close (like over 80-90%) then I'd go ahead with getting 2x4 chips. You don't really gain any speed by having "extra" RAM.
As for 2x4 vs 2x8, I'd go with whatever's cheaper. You'll want the same sized chips either way to make use of "Dual Channel" mode, whether it's 4x4 chips or 2x8. Games are just starting to make use of 16, but I don't think you'll need to plan for needing 32 GB unless you run programs that need it (like software development, media creation, etc).