This isn't even intentional absurdity. The theme of the song is bragging. Here are some other lyrics.
I'm down with Bill Gates
I call him "Money" for short
I phone him up at home
and I make him do my tech support
Your laptop is a month old? Well that's great,
if you could use a nice heavy paperweight
Installed a T1 line in my house
Upgrade my system at least twice a day
The line about having 100 gigabytes of RAM is completely in keeping with every other part of the song. There's no more reason to think Weird Al might not have known what RAM was than there is reason to believe he didn't know that having Bill Gates do your tech support was unrealistic, or that PCs are rarely upgraded more than once a day.