Generally, in my experience:
P-M (Centrino) = Celeron-M (Centrino based) >> XP-M (Barton) > P4-M (Northwood) = Celeron M(P4 based) > XP-M (TBredB based) >> P4-D (Northwood) = Celeron-D (P4 based)
where >> denotes a larger difference.
Haven't yet used/seen how an Athlon 64 fits into this, my guess would be a little better than the XP-M.
I personally have a Compaq Presario 2100 with a xp-m 2400+ (barton 1.8GHz) CPU and get about 3-3.5hrs battery life from a 8-cell battery.