You may want to investigate another CPU, I think that no matter what card you look at, it will end up being limited by the one you have.
I guess you are running a Duron, you should be able to go quite a ways further with that motherboard but you haven’t given much to go on.
I think in therms of getting most for your money, you would be best to avoid buying any of the NVidia FX series cards, they are pretty much seen as a mark time card from the Ti series.
The FX series cards can render DX9, but other than that the Ti series, which mostly don’t run DX9(except the Ti4800 - which is just a Ti4200 with some few extras installed) the best of the Ti series is definately the Ti4600.
You can get these cards very reasonably on E-Bay.
By doing this you could probably save yourself enough cash to purchase a motherboard and chip, something like a AMD XP2500+ would be a good bet, again, you will get something very useful for moderate cost.
If you want to get something that can run DX9, which is what most of this generation games use, then the Radeon9600XT is a very good bet in terms of price/performance, especially if you can get hold of the Fireblade version which is fairly rare.
If you have a little more money then perhaps you might look at the current best price/performance card which is the GeForce 6600GT AGP.
This is running the very latest technology and was originally built for the shiny new PCI-E motherboards, however there is now and AGP version out.
It outperforms all the other AGP cards out there, including the marvellous but expensive Radeon9800XT, and it does this by quite some margin too.
This article is a little out of date, since it has been overtaken with the latest generation, but most of the points you need to consider before purchasing are highly relevant.
http://graphics.tomshardware.com/graphic/20041110/index.html
Still think you’ll need a more powerful CPU though, it really depends upon your cash, some of us think its not too wise to go out and purchase the very latest cutting edge stuff as the value drops rapidly, technology moves on too quick, and they are therefore somewhat of a waste of cash, since you can do just as well for half the cashh, if you are prepared to wait six months after the latest shiny thing comes out.