The only issue with the fatboy PS2 is that if it breaks (and they are know to) you are pretty much screwed and will end up having to move to PC or Xbox. It'll be increasingly difficult to replace. In the long run you may end up saving yourself money and grief by going the PC route.
Tell me the stats on your comp and maybe I'll be able to make some recomendations. Also keep in mind that FFXI is emulated (badly) onto PC from PS2. While it still looks and runs arguably better on PC it's definitely not optimized to where it could for PC. The problem? It runs more based on processor power than on vid card.