I think because of the way FFXI is built (i.e. built for PS2 and then just dumped on PC), it's heavily reliant on CPU more than anything. If anything, more RAM and a better graphics card, are always going to benefit you.

Most graphics cards that can run FFXI really well, are pretty damn cheap now. RAM I'm not sure about.

Anyways, I think it's worth upgrading PC to play FFXI, defenately!