I've not messed around with my X-Bit modded Xbox since I installed it around 5 years ago. Of course I still update XBMC every now and then but I'm very rusty.

I want to slap in a 500Gb HDD but I have no clue what BIOS to use, where to get it from, how to flash it. Can someone tell me?

Unrelated question, my fat32 500Gb HDD already has movies etc on it. Can I just create an Xbox structure with XBMC on it when it's in my PC then put it in the Xbox and it'll work? Or will I have to use that Evox boot up CD I vaguely remember?

Thanks for any help you can give me

You will need an LBA48 compatible bios.

If you create partitions larger than 256GB, you will need to format them with the xbpartionner program.
This is the only program that uses a 32KB cluster size. (or even 64KB)

You can't leave the media on the disk.
The xbox partition structure requires a reformat of all the partitions.

As there are no rescue tools for the xbox fatx file system,
it's best to have a backup of your media "in case off".

regards.