Well, there are some good AGP cards still (with the technology you're best to go with 6600XT, you should still cap frames in areas even like Whitegata AH). But, if you want to go with an AGP card you'll spend more on it. You may be best waiting on buying a motherboard + graphics card at the same time. A decent Motherboard will run you in the 100~150, but with that you'll most likely have to buy a new chip also.
I'm currently running AM2 Chip on dual channel and I really like it for what I do (usually some high-end stuff + FFXI), but it does have a lot of conflicts with older games such as Counter Strikes below source (I actually have to change my rendering from OpenGL to software to make it run properly) and anything rendering in a system below DirectX7. But, with games like Farcry, Counter Strike Source, Doom3 I run @ highest settings non stop without even pushing the card.
A really good choice if you are planning to upgrade would be getting a SLI board. With SLI you can make 2 Graphics Cards act as 1 and get a HUGE system boost. Many more games are starting to offer SLI-friendly programming (although it will run all) that actually will let you get more than 2X the boost on your games.
For FFXI I never see any lag anywhere; Dynamis, Whitegate @ AH, in the Boyadha Tree near water, all run at 30 FPS (cap). I liked my ATI x1800, but ATI just dosent render multiple surfaces half as well as Nvidia. I even dont get the frames cut when I'm around bard songs, etc.
But, yeah AGP is really a thing of the past, slower information transfer between hard drive and really makes any card above 8 pixel pipelines useless.