[Solved] FSB, external CPU speed, quad pumping... what the ..

There are several different buses on the motherboard and some of them can be quad-pumped while others are double-pumped and others yet are normal single-data-rate buses. You would need to tell us what bus in particular you are looking at (memory bus, CPU frontside bus, HyperTransport or QuickPath Interface bus, etc.) as well as the make and model of your motherboard. You gave us that information, so we can identify what buses you have and tell you more about them.

The "external CPU speed" is the system clock, which on your board drives the frontside bus. The AMD Athlon and Athlon XP processors your board support are double-pumped, so that 133 MHz FSB clock results in an effective 266 MHz FSB speed. That does seem slow compared to many later Intel CPUs like Core 2 Duos and Quads running at 1333 MHz effective FSB speeds, but remember that the chips that ran in this board are very old. The contemporary Intel CPUs had effective FSB speeds in the 400-533 MHz range. AMD eventually released Athlon XPs with a 400 MHz effective FSB speed before moving to the Athlon 64s, which have no FSB and instead were directly connected to the memory.

Absolutely not. Both the Athlon XP's EV6 FSB and all of the generations of DDR memory (including DDR2 and DDR3) are double-pumped and move 8 bytes of data per transfer (or 16 bytes per entire clock cycle, since they are double-pumped.) Your chip's 266 MHz FSB can thus move 2.133 GB/sec (8 bytes/transfer * 266 million transfers per second) compared to 3.200 GB/sec for the DDR-400 (8 bytes/transfer * 400 million transfers/sec). DDR-400 is fast enough than no Athlon XP processor made will bottleneck it; the 400 MHz FSB units like the 3000+ and 3200+ Bartons have exactly the same bandwidth as a single stick of DDR-400.

It's double-pumped like your Athlon XP's FSB is, so it only runs at 200 MHz.

It should work. You just won't get any more speed than the IDE interface will give you, so your HDD's speed will likely be limited to 90-something MB/sec.