BEST AGP CARD FOR $100 - $150:
Radeon X800 GT
Codename: R420, 13 micron technology
256-bit memory bus
475 Mhz core, 490 Mhz Memory
Essentially an X800 XT GPU with only 8 pipelines enabled, the X800 GT's high clockspeeds and 256-bit memory bus allows for very respectable performance, beating out the Geforce 6600 GT in this AGP price segment.
BEST AGP CARD FOR $150 - $200:
Radeon X800 GTO
Codename: R430, 13 micron technology
256-bit memory bus
400 Mhz core, 490 Mhz Memory
The only card that currently offers the best performance in this price segment for both PCI-express and AGP, the X800 GTO but boasts a full 12 pipelines, 256-bit memory bus, and high clockspeeds. Known to be good overclockers, these are great cards for the price. In the future Nvidia plans to introduce an AGP version of the Geforce 6800 GS, which would be excellent competition for the X800 GTO, but it is not yet available.
i think you really should upgrade you cpu first... it socks when you play games. i just upgrade my from 1.4 to amd athlon 64 3500. with the same gpu, i have about 20%fps increases when i play half-life 2.
I'm with you. A 1.4 TBird is below minimum spec for many games. AOE3 won't run on that chip (no SSE). Other games like NFSMW and NFSU2, the detail levels will have to be turned down to low reguardless of the video card. Not even talking about fear, COD2, and other hardware crushing games. I just upgraded from a 1.33 TBird in my kids KT400 system and saw a big improvement in framerates in a few of my games, even with a GF4Ti4200 in it. Then put in a R9700 pro also for another good boost. Honestly in my opinion, without a CPU upgrade, I'd look no further than a $75 used 9700 pro or 9800 pro, and a $35 512MB stick.
Anyway, a cheap GF6600GT or X800GT, AXP2400+ or AXP Barton, and a 512MB stick would be a nice all-round boost. Although, $200 isn't going to happen unless it's used parts. Tough to spend the money they are asking now for AXP's though, when you can get a S754 mobo and Sempron 3100 for about the same price as a slower AXP CPU. That combo, 1GB PC3200, and a 6600GT can be purchased for under $300 with some deal searching.