Both above statements are true. The Athlon 1.4 GHz runs on a 100 MHz FSB (double pumped to 200 MHz) and uses 133 MHz memory. The Athlon XP 1600+ also runs at 1.4 GHz, but it runs on a faster 133 MHZ FSB (double pumped to 266 MHz). It also uses 133 MHz memory. This FSB speed increase translates into a 5-15% speed increase for the Athlon XP 1600+ (roughly).
Both chips can be run in modern AMD Motherboards, and both CPU's can be run with either DDR memory or SDR memory (DDR being faster, of course). However, it is strongly recommended that you run an Athlon XP with DDR memory, whereas regular Athlons are a bit more forgiving to being paired with SDR memory - they don't get as much of a speed increase from using DDR as Athlon XP's do.
Altogether, an Athlon XP with DDR memory should run about 10-30% (estimated) faster than a same-speed Athlon with SDR memory.
The Athlon XP is better in every way - I'd recommend buying one of those given the choice between the two.