First you want the i586 download (not sure how Mandriva breaks up packages on cd's though)
x86_64 is for AMD64/Opteron & P4 w/ EMT64
Second yes, all you need to do is set a some open space for linux to reside on.
3gb is fine if you never want to save anything or add anything to it, other words make it larger
No formatting is necessary your linux installer will format it with Ext2, Ext3 or ReiserFS all native linux file systems
Tips & tricks : Read. Google. and Patience.