2x2 will be about 10-15% faster than single stick. However the size is pretty important, so I would take the single 8GB stick. Memory management can be a multitude of issues. Reinstall drivers for one, bad drivers can cause a multitude of BSODs. It could be also caused by corrupted system files, so you'll want to run SFC (command prompt -> type in sfc /scannow). You can use windows to run memory diagnostics as well (Windows+R -> mdsched.exe), but it will probably come up with nothing. You can also use different software that you know of to run diagnostics and such, some of them will most certainly be better than what Windows provides (at work, can't think of any off the top of my head).
If none of that works then it might be time to...