A "sealed" case won't stop it from being upgradable if the case can be in any way cracked open without damaging the contents; the RAM will only be un-upgradable if the rare step of soldering the module directly onto the motherboard was taken, and it is unlikely the hard drive is anything other than a standard laptop ATA. Compared with some of the hacks enterprising users did with the Risc PC like the pizza oven, there is sure to be some level of unofficial upgradabity to this unit, if even it only means replacing the hdd with an 80gb 7200rpm unit. Keen Amigans used to solder wires onto their motherboards to add RAM afterall ; ) What a "sealed" case really means is it'll be obvious when the warrenty has been voided ; )
These processors' stated MHz speed reflect their target function as ultra-low power chips. They could probably be rated for much higher speeds if the tight electrical and heat restraints present in their target market weren't present. What I'm saying is they'd probably at least theoretically 'overclock' very nicely in a RISC OS desktop situation.
Castle would probably prefer to stay with GeForce cards because they only have to modify an existing driver instead of forging a new one when changing to an updated card model. Why go Radeon when they can still purchase GeForces? Likewise for the high end cards: supporting the latest whiz-bang cards' featureset would be a sizable software-side financial investment which might only result in a meagre number of sales that wouldn't have otherwise happened.
A floppy drive + cable cost only about £7. Removing them from the 'Aria' is going to do little to cut the total cost of the unit or increase Castle's margin per unit. Then again, some people just hate floppy drives : )