First, the presence of an AMD 64 chip (likely 939 Chipset) in the HP Pav says enough to suggest this is a PCIe system. I'm also assuming from the use of a GS series card, that it's a low-volt system that's running the older PCIe x16 slot and not PCIe 2.0.
Second, the cards satrn pointed out are overpriced (no offense), and in no way worth the money, along with being absolute overkill for your system and the game you're playing.
I would suggest the following. From my experience with HP Pav's, you likely have a two-slot SDRAM configuration for memory. I'm going to suggest looking up the model number of your HP Pav and find out what size SDRAM slots you have and upgrade to 2 GB of memory. This is sometimes a more important step than upgrading your video card, and can be much less costly.
Since I'm
guessing that your system is not using PCIe 2.0 and is using the older PCIe x16 that was popular with DELL and HP systems back at that time (check the manufacturers model # on your PC to determine the from the specs what the slot is for sure
before you buy anything), I'm going to recommend
This Video Card, which runs about $50 after the rebate. Keep in mind, this card is even overkill, and you can probably go down to a mid 8000 series card like
This Card, which runs around $30, and run FFXI like a champ.
Now, make sure you check and are sure of the slots. The best way to get the specs on your PC is to get the model # (on an HP Pav, should be on the front face at the bottom, such as
a335w, a popular bundle model from back during the 2003 holiday season. Google the model number, and look for reviews or
system specs for that model. Specifically, you want to know what video slot it has (either AGP x8 or PCIe x16) and the SDRAM slot size (ie. 184-pin DIMM). This will give you the information that will allow you to purchase the new hardware needed to update your machine.
The bad news is, your processor is aging, and HP's motherboards (like Dell's) don't allow for a ton of expansion. Eventually you're going to just want to buy a new system, and they're getting cheaper and cheaper. For under $600, you can build a new system from scratch with monumentally better specs than your current system, as well as toss in a new monitor. Use sites like NewEgg.com to price & purchase the parts, and don't be afraid to ask questions here or elsewhere on every step you make to make sure you're doing things right.
Good luck!