Yeah, you had ROM space (and potentially cartridge ROM space) eating into your 64K on all the models and had to bank-switch for anything more than 48K IIRC. With a BASIC cartridge in place you'd lose another 8K.
Also I don't recall any of the 400/800 3rd party upgrades being compatible - they used a different bank-switching mechanism - with the later XL/XE models.
"bank switching" sounds familiar, so I'm pretty sure my memory of 64k is correct then. I can't say I understand what it means, but I remember something of that term at the time.
You group your memory into groups of 16k (could be other sizes, 16k is what I recall but it has been years). Then you have a switch that you can program the switches between groups (also called banks). You can only access one at a time, but you can switch between them very fast. It was possible to get up to 5 MB of memory that way, even though you could only use 64K at a time.
Where did you get a real keyboard for the 400. I searched and searched and could not find one. I was so envious of the Timex / Sinclair folks who had a nice replacement.
