Generally it's good to balance the channels with equal amounts of RAM, but if you have lots of APPS running at once, then performance will be slightly better with 2 + 4. So if you run with 2 + 4 just now, with a view to 4 + 4 in the future...
Always check the speed of the RAM you buy, faster the better as long as your rig can handle it, check your Specs. No harm in buying faster RAM for your upgrade, but it will run at the slower speed of your existing RAM, you will experience the benefit of the faster RAM if and when you upgrade to 4 + 4
Meanwhile check your Specs to ensure that you don't exceed the Maximum RAM allowable, and that you are running 64 Bit Windows. (32 bit is limited to max 4 Gb RAM)