I upgraded my Armada M700 last year. It had a 17gig hard drive, I changed this to a 40gig seagate momentus click here
£45 from click here
It took about 10 minutes to swap. i upgraded the O\S from 98se to XP.
A lot may depend on the bios version of the laptop as to what size hard drive you want to upgrade to. The machine has worked perfectly with the upgrade.