Myself, I have a Geforce 4 MX 440 card and it runs FFXI very well (although it doesn’t support a few optional bells and whistles, like bump mapping). It also only cost me $50 Australian, and that was over eight months ago, so I assume you could pick up comparable cards for around the same price or even cheaper.
As far as I’m aware, SiS video cards aren’t supported on FFXI, (I had a Sis video card on my now defunct, antique P1 200 mhz system), so upgrading is about the only way to run FFXI I’m afraid.
www.newegg.com