I think it would matter what you intend on doing. If you are gaming or video editing at all the faster Ram will help. But i have seen Test with Ram at both speeds and your CPU may never get above 2GB of ram unless you open like 10 apps at the same time. Plus you could always sell the 4GB of ram to some one and buy one 2GB Ram stick and wait till you need more than 2GB of ram and buy another 2GB stick then. I guess it all comes down to what your needs as far as apps you are running, and if you have to see everything go really fast?
I have been running on a 6 year old PC for Video editing, and it is pretty slow for a new iMac user to use. But i am use to it and I don't want to spend the extra cash yet for a new iMac or Mini. I don't think you will see a huge jump with a slower ram.