I'd stick with your current XP-M and go for a newer card, which you can carry onto a new system down the line. In the meantime, you'll enjoy a faster, prettier image on all your games. If you find a X800P/XT or 6800GT/U for well under retail, I'd jump on it. If you're not happy with it, you'll almost certainly be able to sell it and come out at least even.
You can check Firingsquad's recent X800P Athlon scaling article to see how a X800P and 9800XT scale with a range of Athlon XPs and an A64.
Edit: this was in response to the OP, but I suppose it can apply to your Q, too, ptw. If you have the money, why not enjoy a X800P+ or 6800GT+ right now? Remember, the resale value will be higher when you decide to move on in a few years, so it's not a total loss. PEG sounds great, but it'd require a lot more upgrading on your part, which would probably not be worth it for the performance gain you'll see going from AGP to PEG.