MacBook – What do I need to know to buy an SSD drive for a 2008 MacBook 5,1

Should be similar to the one I got for my MacBookPro, actually here is one especially for iMacs: OWC Data Doubler

I got another one from ebay for ~20 $ - does not fit perfectly, but is good enough for me. Up to you which way you go.

MacBookPro installation was not too difficult, don't know about difficulty for iMacs.

It is a variant of Mini PCI Express & mSATA:

Some notebooks (notably the Asus Eee PC, the MacBook Air, and the Dell mini9 and mini10) use a variant of the PCI Express Mini Card as an SSD. This variant uses the reserved and several non-reserved pins to implement SATA and IDE interface passthrough, keeping only USB, ground lines, and sometimes the core PCIe 1x bus intact.[11] This makes the 'miniPCIe' flash and solid state drives sold for netbooks largely incompatible with true PCI Express Mini implementations.

Source: http://en.wikipedia.org/wiki/PCI_Express#Mini_PCI_Express_.26_mSATA