Getting more memory then 4GB is going to be expensive, anywhere you go to. Since 4 GB is what you get in all iMacs that are new. There is no need to upgrade unless you want to spend lots of money on DDR3 RAM
Getting the referbs are cheaper but yo...
Two 2.8GHz Quad-Core Intel Xeon
8GB (4x2GB)
500GB 7200-rpm Serial ATA 3Gb/s
NVIDIA GeForce 8800 GT 512MB
Two 16x SuperDrives
2x Apple Cinema HD Display (30" flat panel)