Q: Is iMac9.1 mid 2009 or early 2009?

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.

Yes, the Early (or March) 2009 9,1 iMac supports 8GB of RAM.

see > iMac (Early 2009) - Technical Specifications

For quality RAM upgrades, visit either OWC or Crucial

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.