It seems that it's not really a matter of quantity of RAM at all it's an issue with who has access to the RAM, it seems that the PS3 doesn't like it when RAM is shared (a given app can reserve a set amount of RAM and that doesn't change), which makes sense since having one app peak into the RAM of another app is a sure fire way of allowing hackers to run amok.
As for quantity vs quality in terms of RAM it's hard to say, games that really need large amounts of RAM over long periods of time tend to be open world games (where an entire landscape needs to be loaded up), fast pace games like FPSs tend to load items in to RAM for a short period of time before discarding it (i.e. it only needs to load an enemy model until you've blown up said enemy model then it needs to load another). Of course ideally they'd have large quantities of fast RAM but then you'd see costs sky-rocket.