Best ram configuration?

I looking to upgrade to 8GB on my 21" imac from late 2009. so I've decided owc (http://www.macsales.com/) might be a good choice. So from what I;ve seen I have two options, get x2 2GB sticks or one 4GB sticks. Is there a better option? The 4GB stick costs about 10 dollars more but would be more beneficial in the long run because I would be able to upgrade to 10 or 12 in the future sine there will be one more slot open if I wanted to, or but the 4GB stick in a new computer. Or is there a problem or performance issues with having 3 slots only occupied? Thanks for the help

 

Thanks for the advice, reassuring. Yes I remember something about dual channeling but it doesn't sound like it would reap much benefit