I bought 8GB of Mac memory from Kingston based on some deal I found online. It cost $87 and I sold my used 4GB of RAM on eBay for $20. So the upgrade cost me $67 total. Pretty good deal, in my opinion. I've had the RAM installed for over a month and I haven't had any issues.
EDIT: Never mind, I see you already bought some. But still, you can get around $20 on eBay for your old RAM.