It shouldn't be a problem if its on a separate memory channel. However its probably going to be cheaper to get another set of 2x2GB than a 4GB stick by itself.
I really just want to add one additional chip of memory because the i7 processor runs with optimal bandwidth when you only have three chips of memory. If we are 100% sure adding a bigger stick of ram (4,6 or 8gig) will work along with my 2 2gig sticks then that's the route I will go. If not then I'd just add one more stick of 2 gig ram and be satisfied with 6 gigs total.
I'm sure I'm just being overly cautious here but you said it "shouldn't" be a problem if its on a separate memory channel. It will be on a separate memory channel so do you mean it 100% sure will not be a problem or just you don't think it will...
Sorry, I just don't want to waste time/money with out being sure. :-)
i assume you are running dual channel RAM for your i7 mobo (i.e. LGA1156). Since you already have a matched pair of 2x2GB, your best solution would be to just get the exact same pair if possible and plug it in. That way you have a total of 8GB of RAM, all running at the same speed and latency. You have to realise sometimes capacity isn't everything and that the latency has a part to play. Unless you are doing intensive animation/media/graphics design kind of things which would really benefit from the added RAM. if not then just stick with 2x2GB
If you do insist on getting another stick on it's own, check your motherboard manual to see which slots should be used first. trnddwn33 is right, because it's on another channel, the RAM will work, whether it works efficiently and together with the existing RAM is another issue that you should look into. For the i7, I have found very few articles/reviews/experiments that have handled mixed sizes/types of RAM on the same board. I hope this helps! =)