AMD Athlon XP's are not 64 Bit, also the 2800 is considerably slower then a Athlon 64 @ 3700. the X2 have 2 cores, which makes it great for multitasking and encoding.
PC2700, is slower then PC3200, PC3200 is DDR 400 while PC 2700 is DDR333 (i think)
Basically memory at PC2700 is slower then PC3200. Memory speed is measured in Mhz.