Pretty much, sadly there are certain compatibility issues with newer nVidia cards that make the game run worse or only as good as the above old p3 I've mentioned. I remember upgrading its video card to a GeForce Ti 4200 back in 2004~ which simply kicked ass.
FFXI's on an old engine though(8~ years directx 8?), can't really blame it too much I suppose.
From what I can tell, the GeForce 7xxx series was the last set of nVidia cards that really ran FFXI well. After the 8xxx series came out, there were all sorts of glitches that took several driver revisions to iron out, and some of the performance problems never really went away.
There's a post in an old thread at: VN Boards - AA/Aniso not working with FFXI which explains some of the FFXI engine's oddities, like rendering to target, which are increasingly less well optimised these days. It's also the reason why hardware Anti-Aliasing never worked.
My problem it with an ATI card, not an nVidia. The Radeon 9600 Pro runs the game horribly ever since WotG. Problem is I upgraded from a GeForce MX4000 to that just for C&C3, and better dual monitor support, and that card is ANCIENT. Omega drivers gave me about a 2-3 FPS increase, and POL tech support says the last driver they support is from April of 2005. I have an AGP motherboard, so a different card is hard to find, and I don't really game on my computer enough to justify a new one or the cost of a new graphics card.