Well, for now, I'm using the 4MM_RD padstack, with a width of 15.75mils(0.4mm).. It's the same padstack used by the OMAP-L138 footprint that I'm using provided by TI. The balls on the L138 seem to have the same dimensions(or at least within the same range: tolerance levels of 0.55/0.45 on the L138, while the DDR2 IC only states ø= 0.45) with the same pitch(0.8mm).
Someone please let me know if I'm way off. (I'm also fairly new to PCB design in general, as you might have guessed)