Indeed, DDR has much more bandwidth and higher FSB speeds.  I do think it is backwards compatable, for example, 333 FSB RAM would work with 266 FSB RAM.  Which leads to 400 FSB would work with 266 FSB RAM IFF RAM voltage is the same.  Going the other way, I recently read of a person who overclocked an AMD 2500 Barton to 3000 by changing the FSB to just over 400.  With PC2100 RAM!  And stated a stable system.
On CL speeds: I've read the pros and cons.  Some say not significant.  For my system the performance increase was significant.  Clearly CL2 was a better choice.  I note that THG tests of graphics cards using benchmarks, games and FS2004 use 512 MB Corsair best performing RAM.  CL2 is not available for the AMD 3200 CPU.  My next upgrade will likely be to an AMD 3000, 333 and I'll overclock PC2100 RAM to match.  THG tests show that the difference between the 333 and 400 MHz CPU is small and price gap is large.

I'll check the underclock of RAM out when I actually execute.  I may just turn over the current CPU and RAM to daughter and buy both new CPU and RAM.  Future stuff.