I dont mean to moan at you m8, but seriously you cant get a decent gaming system by adding a decent GPU to a system with 256MB of RAM. You need to understand hardware infrastructure, adding a decent (512MB ATI card for example will cost you £50) to that machine is POINTLESS.
256MB RAM was adequate two years ago, modern systems really need 1GB and that includes Linux OS's.
That PC will be hopeless, the customer is better off spending £250.
This idea is pointless. Seriously dont bother. I'm not trying to be nasty at all. But on this, you're just wrong.
Seriously drop it, and think of something new.
And no it's NOT fully upgradable! it couldn't be, you'd need a motherboard costing more than the PC to be fully upgradable.
Stop fibbing, be honest it'd be a crap piece with a cheap price tag.
OO is no match for MS Office, Linux is not as mature as XP (let alone Vista and OS X).
Sometimes you wake up and think you've got the next best idea, and sometimes it turns out you're wrong. a lot of us on OCuK know our technology domain, listen, think and discard this idea. Trust me, stop wasting your time.