Since you ask, I will gladly indulge in a little pedantry.
2.5 is pronounced "two point five", that is "two and a half".
2.048 MB is pronounced "two point oh four eight megabytes", therefore:
2.048 MB
= 2 MB + .048 MB
= (2 MB * 1024 KB/MB) + (.048 MB * 1024 KB/MB)
= 2048 KB + 49.152 KB
= 2097.152 KB
Not necessarily, it depends on the punctuation used to group the digits in a given country, where a dot or comm can be used either to group the thousands or the decimal places.
In the present case, the dot is used to group the digits so that the thousand is seperated from the other numbers for easier reading, while the decimal places would be seperated by a comma.
Therefore, Marty is actually saying he now has 2.048,00 MB of RAM. Since the decimal digits anyway, it's ok to put 2.048 to signify two thousand forty eight.
If he put 2,048 however, that would be a different story (provided, of course, that in the given country where he is the comma is used to separate the decimal places).