If you S939 you must buy a PCI-Express capable mboard which may impact on your budget as you'll need a new gfx card as well (assuming you're upgrading not starting from scratch).
This will give you the greatest longterm flexibility and gives the longest shelf life of the PC you build.

However if you want to stick with AGP to save money I'd save more and go S754 and the Sempron 3100. An S754 mboard will still allow you to upgrade to a 64-bit processor later at a current max of A64 3700. It minimises your initial outlay while guaranteeing a minimum level of future compatibility.
It is not the most future proof but works well with a limited short and long term budget.

I chose the second option at the end of last year when it made even more sense. Going PCI-Express was not an option at the time for A64 owners and I'd recently purchased a Rad 9800pro and so prefer to stick to AGP. Next time I upgrade will likely be when the A64 3700 S754 hits a much more attractive price point.

Hope you enjoy the build!