Well I summed up my options. I decided that I didn't really need 500GB. Also I wanted to grab a HDD from a Seagate FreeAgent Go and some of the 500GB versions used a Samsung disk. Not what I would want.
Bought a Seagate FreeAgent Go 320GB for AU$90.25
Took out the disk. Very easy, just pry the top off and pull the disk out.
Re-imaged it with the 320GB (5400RPM) disk in my N10.
Built-in windows tools made it easy as pie.
The first thing I noticed was that it my N10 has become a lot snappier. I haven't run any defrag yet so it might be even quicker.
I can't comment on battery life as yet but both the disks were rated at the same current (yes I realise this is the max current draw but I assume they would use a similar amount of power).
Overall very happy. And I also now have a 160GB portable HDD.
P.S It is VERY hard to find any Seagate 500GB 5400RPM drives. Pretty much every shop sells them as soon as they get them in the shop, so if you plan on getting one in the Melbourne area make sure you find out when their stock comes in and buy ASAP.
N10Jc - 2GB RAM - 320GB Seagate 5400RPM HDD - 3.5G BT Modem - Win7 HP - AND stickers!