http://support.apple.com/kb/HT1758 from this article, there is no Mid 2009 model.
I have a iMac9.1. Does that support 8gb RAM?
iMac, Mac OS X (10.6.8)
If you go to http://www.everymac.com and look up your iMac9,1 it shows that machine will support 8 GB or ram. There are a lot of 2009 models so you will want to find the exact match but the list says 8 GB should work.
Thanks Ralph Landry1, would you suggest me to have one 8gb or 2x4gb?
Thanks again
Yes, the Early (or March) 2009 9,1 iMac supports 8GB of RAM.
see > iMac (Early 2009) - Technical Specifications
The iMacs always seem to work best with matching pairs of ram modules rather than a single module. I lso would recommend the Mac-friendly sources den has pointed you to...OXW and Crucual...have used both a great deal and they give great service, lifetime warranty and quality ram...keep in mind Macs are notoriously picky about their ram so get memory from a reliable source.
Is iMac9.1 mid 2009 or early 2009?
