Asus P4T533-C Motherboard for intel Processors OEM uses 16bit RDRAM (which means you need 2 rimms for it to work) and comes without manual, cables or anything else.
Asus P4T533-C Motherboard for intel Processors, Retail is the same as above but comes with manual cables and everything
Asus P4T533 i850E Motherboard for P4 Processors Retail uses 32bit RDRAM which can be used alone, in other words you need a single RIMM to make it work. It's also packaged with manual & cables.
Whichever you choose, make sure you pick the right memory because 32bit RIMMs wont work in a 16bit board. About memory size well I'm running Win98SE and I have only 256mb of PC133 SDRAM and my father is running a P4 1.5 with Win2K and he has only 128mb PC600 RDRAM. However, the P4 is slowwww and it only gets worse when multitasking. Id say get as much as you can, while 256mb would be ok, 512mb wont be too much if youre running WinXP!
Oh also the OEM mobo has a much shorter warranty while the other 2 most likely have a 1 year warranty.