System Upgrading: Help on Decisions

I have been a forum lurker for some time now, but need some assistance now with a new project.

When I upgrade my computer, I tend to do so piece by piece, phasing out the slowest parts for new ones a little at a time. I am looking to do some more upgrading soon and currently have this core set-up:

Win XP Home SP2
Athlon XP 2400+
Asus A7A-266
1 GB PC 2100 (2X512)
80GB AND 40 GB ATA HDD
ATI 9800 PRO 128MB
SOUNDBLASTER AUDIGY 2

I was thinking a processor and mobo are the weakest liks in my current system and was looking to replace them next, but I came up with a couple questions while shopping:

1. With the Athlon 64, FSB speeds say " on chip". Are all versions of the processor able to run at the same FSB speeds?

2. Is it worthwhile to go with a socket 754 or is it better to go socket 939? It seemed that the newer, faster chips are mostly the latter.

3. Any suggestions on a motherboard? I was looking at a MSI K8N Neo 2 Platinum. IS it better to stick with AGP or should I leap to PCI-e and get a new graphics card too, once I pay Uncle Sam this year?

I would prefer something i can reuse most of my other components for instead of replacing alot at once, but I don't want to shoot myself in the foot when theying to upgrade again in another 6-12 months. Thanks for all your help in advance folks.