One thing to keep in mind when talking about SATA3 SSD's in SATA2 machines is that the SATA2 "bottleneck" only really comes into play in large sustained transfers. However, most of the WOW factor of SSD's shows itself in the very fast, random, small transfer size (e.g. 4K, 8K, etc...) which are not anywhere near the limits of the SATA2 interface.
So buying a newer SATA3 SSD with better 4K (IOPS) will still benefit your performance in a SATA2 machine. I have two late 2009 27" iMacs (SATA2) with Intel 330 180GB SSD's (SATA3) in them and I can not tell the difference in day to day from my 2011 Mac Mini (SATA3) with Vertex4 (SATA3). Sure, the Mac Mini benchmarks faster using BlackMagic but in terms of noticeable difference in day to day use... I don't see it.
IMHO, Get yourself the fastest affordable SATA3 drive for your machine. Don't worry about trying to find older SATA2 drives. The one caveat to that is that the 2009 machines that use the 9400m chipset have difficulty with some Sandforce SATA3 drives. So if you have that chipset the safe bet is to use non-Sandforce drives.
Edit: and to answer your question about the Samsung 840, my son has the 120GB version in his 2011 Macbook Pro (SATA3) and it is very fast. If you can afford the 250GB version the write speed is much better so that would be a nice upgrade... but you would be pleased with the 120GB Samsung 840.