Just got a new iMac myself this week with 4GB and looked into the very same thing. I was told you don't need pairs these days and so went for the single 4GB card from Crucial for about £35 inc postage. This still leaves a spare slot for later. Made a big difference in speed. Bit cheaper than Apple's upgrade quote of £150! Absolute doddle to fit in about two minutes.
Cheers, dougie