[Balloon] Balloon3 - how much RAM?

Another quandary, chaps.

To get the power low in Balloon, it's time to start supporting low-
voltage, mobile-oriented SDRAM. All well and good. However, it's
also available in BGA, which would let me save some space - also
good. (In fact, quite a lot of space - a TSOP54 package, like we
use on Balloon2.0x takes 312mm^2, and an 8x11mm FBGA takes
130mm^2. However, you can't stack BGA packages like you can
SOP, for the real density obsessives. Current RAM seems to offer
up to 256Mbit / package in mobile form, 512Mbit in 'regular'
SDRAM.
Balloon2.0x allowed us to fill all 4 banks of the SA1110
(128MBytes each) for a total of 512MBytes (in theory - I don't
think anyone ever built one that massive, did they?).
If Balloon3.x only supports BGA packages, it'll be limited to a
depressingly low 64Mbytes with current parts. It seems to me that
this isn't enough headroom. I could fit 2 pairs of BGA devices, but
that seems futile. - unless we expect the future to bring us
mobile SDRAM, in BGA, in 512Mbit or above. Anyone got a
working crystal ball I can borrow for a while?

Upshot - I currently favour TSOP 54 packaging, now that mobile
die are available in that package. It's not the best of all worlds,
but I think the flexibility is handy. Would anyone suggest a
different tack? (Socketed memory / SODimms are right out, before
anyone suggests it).
Note that this doesn't affect the NAND / Boot flash in any way -
just because PPC people call everything 'memory' doesn't mean
that we have to...


Steve