does anyone know anything about these things, specifically the version to buy and a reliable source (i'm not after dodgy games, just the card itself)?

also whats involved with transferring existing games onto them (i'm sick of turning the house over to find cartridges that my kids have 'lost' so figure it will be easier to have a bunch of their games just on one card and the originals in a safe place), is it fairly straightforward?

I've got a few spare ones, just load the correct drivers and software to get them to work. Games are loaded of PC onto Micro SD card then you plug the SD card into the R4 card

Not sure how you load original games on to them though