Do you have a Core 2 Duo or Core i5/i7 '09? Annoyingly, with the 27" Late 09 iMac, there were two separate logic boards that were used for the C2D and i5/i7 versions, as they used different sockets. Upgrade options somewhat vary between models.
CPU
With the C2D model, you are limited to the E8600 (dual core 3.33GHz).
With the i5/i7 model, you can upgrade as far as the i7-870 (the 880 is supported but rare and expensive), which is a quad-core clocked at 2.93GHz.
Memory
With the C2D model, you are limited to 16GB (4x4GB DDR3 1066MHz / PC3-8500). 8GB sticks are not supported.
With the i5/i7 model, you can run up to 32GB (4x8GB DDR3 1066MHz / PC3-8500).
In terms of a new graphics card, both use the MXM-B standard, and a new Metal-capable card can be installed as long as it has been Mac flashed (otherwise you get no bootscreen or boot options).
I suggest reading this thread about GPU upgrades for these iMac's.
All SSD's should work, I have personally had good experiences with Crucial, Kingston and SanDisk drives in other Intel Macs.