All that stuff is fine to ask here in my opinion at least.
The max RAM a PC can take is a function of the number of RAM slots and the OS. If the OS is XP or earlier, then 3.5 GB is about all that can be addressed by the computer so even if you had 4 slots with 4 gig sticks each for a total of 16 it would still only count as 3.5 GBs. If you have a later OS than XP, then the max is probably 4x the number of RAM slots on your motherboard.
Those should be easy to count if you look in the case. They are long about like if you stretch your thumb and index finger as far apart as they go and about half as thick as thumb. There are usually 2 or 4 of these, but other numbers are also possible.
Generally speaking, you can upgrade anything you want. Power supplies don't need drivers are anything, as long as the power supply has connectors for everything you need and enough power for everything you need then there is no problem if you upgrade to a newer or bigger one.
You will need to see what connectors your motherboard needs (most likely a 24 and a 4, but possibly a 20 instead of 24 and an 8 instead of a 4) and make sure the new PSU has those. Video cards also have requirements, good ones will need either 1 or 2 of either 6 pin or 8 pin connectors or 1 of each. You would need to make sure your new PSU had enough of those as well.
Also, you have to make sure you have enough 4 pin connectors to connect all your CD drives and hard drives.
Its kind of a pain to keep track of all those things, but for the most part any good PSU has enough for any reasonable scenario. If you want like 10 hard drives you will have a tough time finding one, but with 1 or 2 hard drives they pretty much all have the cords you need if they are good brands (antec, seasonic, corsair).
Same thing for video cards, you would need to get one and make sure the drivers match the OS you are using. If the card you want has no drivers for the OS then it will be wasted space, so if you have XP make sure the card comes with XP drivers or if you have Windows 7 make sure it has windows 7 drivers. Other than that, the cards will probably all be PCIe and you should have PCIe slots on pretty your motherboard if the computer is pretty new.
There is no real difference if you move to a larger case other than the physical size of things. The motherboard slots where the screws go are pretty standardized so any motherboard should fit in pretty much any aftermarket case unless Acer is one of those proprietary PC makers that just likes to screw with people. (Dell moves the plugs for the power cord on the PSU around so they don't fit in regular cases, for example).
Not sure if Acer is like that or not, but quick google searches should be able to confirm or deny that for you.
Anyway, hopefully that is enough info to get you started.