I'd suggest you look for the X3.3294 bios - it was designed to work with the X3, it supports F only (06) and F&G (67), and gives you a whole pile of useful features - in particular X3 config live which gives you an on-screen UI for configuring the BIOS on the fly, has an FTP server for transferring files to and from the Xbox, and generally makes life so much easier if you mess up your config....!
Have a look
here for basic flashing stuff (and follow some of the links on that page for more detail), and
here for a slightly more complete guide. Note that you won't find the bios anywhere on either of these sites.
Make sure you use a standard video cable when flashing (flashbios on the X3 gives a blank screen if you use anything else). When you get the bios loaded DON'T set a password - it doesn't work properly, and locks you out. I was caught out by both of these.......!
You can then use X3 config live to format your new hard drive. Use the inbuilt ftp server to put your dashboard and apps where you want them (I put mine on the E drive) and set up the bios to point to your dashboard. The menu is fairly logical. Then you can select "reboot to dash" to check your settings. If you've got it wrong you can go back to X3 config live and have another go as often as you like!
BUT, as previous posters have said, make sure you use XBPartitioner v1.1 to format your G drive before you put any data on it - the X3 bios formats it with 16k clusters, and that doesn't work for a 500 gig drive.