late win98 games from about 2000 to 2004, but running on a Tualatin 1400
That is a pretty big timeframe.
We're talking about a range between No One Lives Forever 1 and The Chronicles of Riddick: Escape from Butcher Bay. A lot of development happened in these few years.
I think it won't be possible to play all of the games in that range on the UXGA resolution. I have a Tualatin 1.4 GHz and it is very limiting on late games.
My two favorite cards are the GeForce 4 4800 and a Radeon HD4650 (for XP), which is one of the fastest AGP cards.
In some games, the Radeon HD is much faster than the GeForce 4. Far Cry runs buttery smooth with the HD and somewhat acceptable on the GeForce, for example.
But not all of the games benefit from the card. On some, I can't make out a lot of difference between them.
The limiting factor is the CPU and the SDRAM. Don't expect magic results. Games around 2002/2003 should run really well, 2004 games can mostly run with lowered details and in 2005 it starts to get really problematic. Scrapland is barely playable on both cards even with the CPU overclocked to 1.6GHz and it really needs a stronger CPU.
You might benefit from a 6800 in some special occasions, e.g. The Chronicles of Riddick's highest graphics mode was specifically built for that card and Prey runs really well with it, too. But in my opinion, it's not worth it to get one of these, since you can't use them to its full potential on a Tualatin 1.4. Also, later models than the GF4 don't have support for palettized textures and you might need that.
(I mostly bought the Radeon HD card to experiment with AntiAliasing and H.264 decoding and it is doing its job in that regard. The GF4 is my main beast and doesn't need an upgrade.)