I second the idea of using everymac.com as as good resource regarding the memory question. They (and other web resources) indicated 6 GB as a max for my 24" Early 2008 iMac, and that's what I put in (one 4 GB stick and one 2 GB stick). It's worked flawlessly for years.
Regarding the SSD upgrade: I put a 2.5-inch SSD into my iMac where the original 3.5" HDD was, and a 2.5-inch HDD where the iMac's optical drive (superdrive) was, and made my own Fusion drive. It's been great!
I had intended on using the 2.5" to 3.5" adapter that came with my SSD, but it wouldn't fit the iMac's layout. I wanted to finish the upgrade right away, so I ended up using some adhesive foam mounting strips to stick the SSD onto the iMac shell in the 3.5" bay. It seems to have worked fine for a year, now. These strips were thin foam (maybe 1/8-inch) with "permanent" adhesive on both sides. I've also read of people using velcro strips somehow.
Since the SSD isn't all that heavy, and doesn't vibrate like a traditional HDD, I'm not too worried about having super secure mounting. I'm sure it is possible to find 2.5"to 3.5" adapters that would work in the iMac, though.
My 2008 is my daily use machine and I'm very happy with it. Hope your upgrade works out!