1) Does the SD card need to be ext2 formatted, or will FAT32 be OK too?
Assuming it's the same as the other machines, for flashing it has to be fat formatted (fat16 too I think). For everything but the 5500/5000D, an SD card or a CF can be used.
2) in the flashing instructions on the wordpress pages, initrd.bin is mentioned....is that the opie....img file that needs to be renamed or is it something/somewhere else? where would I be able to fetch it if that is the case and where is the initrd.bin I need?
Yes, if you want to use Opie, then rename the opie..img one, if you want gpe rename the gpe one, etc. (not sure if this image is available for you yet though).
3) can I use the latest zImage file from the upgrades or do I need the original, april 28 zImage?
many thanks!
Use the latest one from the upgrades feed - make sure you use the right one (right machine name) and rename it correctly.
Si
P.S. Check the md5sums of the files too (might not be possible for the upgrades kernel, but this is relatively small, it's normally the larger files which can become corrupted when downloading).