Questions about DDR RAM

I'm planning to add some memory to a PC. The motherboard supports "DDR up
to 333 MHz (enhanced) memory bus."

333 MHz (PC2700) or slower DDR SDRAM isn't easy to find on today's market,
and when you do find it, it's considerably more expensive than the DDR
which runs at 400 MHz (PC3200). What would happen if I bought 400 MHz DDR
RAM and installed it in a motherboard which only supports speeds up to 333
MHz (PC2700)? Would it work? If it did work, would it work as well as the
slower PC2700 RAM?

I've gotten conflicting answers to this question. Two computer stores told
me that it wouldn't work. Another store and a book I consulted said it
would.

I'd also like some opinions on which brand of RAM I should get. Kingston
and Samsung are ubiquitous where I live, so the choice comes down to one
of those. Both brands are about the same price, with the Samsung being
marginally cheaper.

Finally, would a 128 MB stick of 266 MHz DDR RAM be able to run in the
same computer as, say, a 512 MB stick of 400 MHz DDR RAM? If so, would
using both at the same time degrade the performance of the faster RAM?

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