Just get the 1x8GB stick and buy the second stick later this year. Techies love dual channel memory, but in real life use like games and productivity apps, it only translates into about a 2% speedup over single channel memory. I actually plugged my memory in the wrong slots, and for 2 years I ran with it in a single channel config. I eventually noticed the error while cleaning out dust from inside the case, put the memory in the proper slots for dual channel, and... noticed absolutely no change.
The only place where dual channel makes a significant difference is in memory benchmarks. Which I suppose might be relevant if you're doing some sort of memory-intensive data analysis or scientific programming. But for general use, you're not going to notice the difference between single and dual channel.