Okay in one of my older machines, I have a 6800GT 256mb AGP I bought a long time ago, but I just recently got a free Radeon 2400XT 256megs. Which is better?
Due to circumstances beyond my control, I replaced my 6800GT AGP with an 8400GS PCI-E for a day or two (I have one of those funky dual-graphics mobos), which I believe is roughly equivalent to/a little better than a 2400XT. In benchmarks it was about 2/3 as fast as the 6800, and minor in-game testing bore that out.
On a side note, I swapped the 8400 out for an 8600GT, not expecting much. Reviews and forum opinions had been saying that a 7600GT was about the same as a 6800GT, then they were saying the 8600GT was about the same as a 7600GT. By extension I believed that the 8600GT would be a small boost at best. It was more like a 15-20% improvement, not bad at all for a stop-gap card that is smaller, runs quieter and cooler and draws less power (as far as I can tell).
I had a similar experience going from a 6800GS to an 8600GT (PCI-E). There was a marked improvement in 3DMark scores (2700 -> 4200 both cards overclocked). From a playability standpoint the improvement was not noticeable and the 6800 just "felt' better in BF2142. It's all subjective but I ended up selling the 8600 on ebay. The 6800GS has been the best card I have ever owned for the price (okay, Ti4200 was great too). The only thing that has enticed me to upgrade is the 8800GT. To me, the 8800GT seems a reincarnation of that formula - value and performance. I will have to wait for the VALUE though