Help with AMd/ Geforce settings

Hi
I've tried various settings for my pc to run FF10, but even though it says that I'm getting 36-40 fps it still moves ...erm.. it feels very very slow and twitchy.
I've also read the settings post and tried to use search on this forum but I'll need some help with this one.

I have :
CPU AMD Athlon(tm) 64 X2 Dual Core Processor 4600+
GeForce 9800Gt 512 mb
4 Gigs of RAM (although XP only supports 3.5 gb of ram)

I'm pretty sure that this game can run on my PC as on my brother pc ( 7600 gt geforce, xp, 2 Gb ram and Intel dual core) it runs perfectly fine.

So if anyone could help me to configure FF10 to run properly I would be very thankful.