alexh: You owe shoggoth a drink. May I reccomend this fine vintage from my cellar?

EDIT: The Falcon sounds a real interesting bit of kit, I'll scour the web for more info!
As for energy efficent accelerators I'd say a Blizzard 1260 with 16MB SIMM would be least hungry. The '060 is 3.3v as opposed to the 5v '030 and '040. Not only is the voltage lower but it doesn't require an energy sapping (and noisy!) fan to cool it.
A newer SIMM of 16MB or more may well have 4k refresh DRAM chips which are less energy guzzling than older SIMMs using 1k or 2k DRAMs (although 1k/2k DRAMs have higher refresh speeds - 16ms/32ms
vs 64ms). Laptops of the time would have used 4k refresh DRAMs.
Going up to 128MB/256MB though would probably be eating into the power!
;-)