Samsung, Intel, Crucial are all good choices. The SATA2 limitation only comes into play on sustained transfer speeds (big files). Most of the "WOW" factor of an SSD comes from the very fast small file transfer rates. Those are unaffected by the SATA2 bandwidth and will be better on the newest SATA3 SSD drives and you will reap those benefits.
FWIW I have two 2009 27" iMacs with SATA2 and they both have Intel 330 180GB SSD's in them. Very pleased with performance. I also have a Mac Mini 2011 with SATA3 and a Vertex 4 in there and I honestly don't notice the SSD being faster in real life other than if I benchmark it it with Black Magic. But again, that only tests sustained transfer rates.
Bottom line.... don't worry about it. Get a SATA3 SSD for the best price and you will not be disappointed with the results in a 2009 iMac.