How do the PCxxxx numbers on RAM map to the DDRxxx numbers?

DDRxxx is in MHz, PCxxxx is in Mbytes/s. Modern chipsets transfer 8 octets per bus transaction (some have two buses), so DDR400 = PC3200 and so proportionally. (Older, two or three digit, PCxx/PCxxx numbers such as PC66 or PC100 were MHz, confusingly enough.)

How do they relate to FSB.

The FSB is also measured in MHz and 8 octets wide. If you have an 800FSB and one bank of DDR400 memory, then your memory performance will be no better than with a cheaper 400 or 533FSB. If you have 800FSB and two banks of DDR400 memory, then you can saturate FSB and your memory still keeps up. Bear in mind though that, in a gaming machine, some of your FSB (up to 2100Mbytes/s for AGP8x) is spent talking to the AGP and not the RAM, so it's not necessarily a problem if your RAM is a bit slower. Modern chipsets, unlike old Triton-era stuff, usually don't require the FSB and memory speeds to be synchronised in any way (though I imagine you get better performance if they are). Check your motherboard manual.

If I get an Athlon 3200+ apparently, I need to use PC3200 RAM. Where can I find some big sticks of PC3200 RAM? I'd like to fully populate this box with 2Gb. Crucial only seem to have it in 512Mb sticks.

If Crucial don't make them, that's IMO a big hint that they can't be made reliably. Big sticks, even in slower speeds, tend to only be available in registered DRAM (i.e. with another set of latches to reduce bus loading), which needs specific chipset support and is not pin-compatible with normal DIMMs.

Peter