I have an ECS P4VXASD2+ (V5.0) motherboard, which supports a 533 MHz FSB.
I want to put in a Pentium 4 3.2 GHz processor (Socket 478) with an FSB of 800 MHz.
Would this be possible? Would the FSB of the processor just be limited to 533 MHz?
There are no CPUs that run on a 800MHz FSB that are on the compatibility list for that motherboard as far as I can tell. This usually means that the computer will not POST with the unsupported chip in.
Occasionally a motherboard will boot with an unsupported CPU, but I would be very cautious as this is usually unstable. In this case, the CPU would run at 533MHz FSB, which in-turn will lower the operating frequency of the CPU. If you need a primer on how FSB speed and CPU speed are related, you can read my answer here, which describes this in detail.
Many overclocker-friendly motherboards allow you to manually set the clock frequencies for your CPU. If your motherboard allows this, you should be able to underclock the CPU to a 533 MHz FSB, and either use the stock multiplier or--if you're lucky--get it to work with a slightly higher multiplier. You probably won't be able to get the full 3.2 GHz out of the CPU, however.