Discharge profile of a SLA battery/5V LDO

I'm starting to work on designing a power supply system for my rover.
I've chosen to provide two batteries, one for logic and one for motors.

On the logic side, it seems like the best way to provide +5V for a
number of circuits is to use a 5V LDO (I'm not smart enough to build a
switching regulator). I don't know my current requirements, but I expect
that 200-300ma would give me enough headroom to grow the design as
needed. I was looking at an LDO like the following:

Max 667
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1143/ln/en

The max dropout voltage is 350mV, so as long as my battery is at 5.4V or
more I should be fine, right? So my question is, is there any indication
how much juice a battery will have left when its at 5.4V? I was thinking
about using a 6V SLA, in the range of 2-2.5maH. But if a 6V SLA is going
to drop out when it still has, say 33% charge left, then maybe I should
move up to 8V?

Does this make more sense than just using a 5V SLA, or can most TTL and
CMOS chips handle the the varying voltage as the battery discharges? I'm
thinking specifically of a couple of small AVRs and a few supporting ICs.

I should add that I was planning on using a 6V SLA to power the servos
and drive motor. I know 6V is on the high end of a servo's tolerance but
should be okay, right? Even if the battery is at 6.4V after being
freshly charged? I picked 6V because of the increased speed and torque.
It seems to me that having two identical batteries would simplify the
charger circuit, is that true?

Also, I had a nutty idea. If I have two identical batteries and one is
draining faster (i.e. the motors), what if I monitor the cell charge
with an AVR, and then switch the batteries when one is nearing
depletion, so that the one that is low runs the logic (lower current
consumption) and the one with more charge left runs the motors. Is this
a crazy idea or has anybody actually seen this used before?

Sorry for the excessive number of questions, it's 7 AM here and I'm a
little delirious still.

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mehaase(at)gmail(dot)com