Reported memory usage doesn't tally with top
 

I have an Ubuntu server which seems to be using more RAM/swap than the listed processes would suggest.

In htop, these are the stats:

Mem = 653MB of 1006MB
Swap = 474MB of 2498MB

Why is swap being used if there's free RAM?

The greediest process is taking 17% of RAM. How can I total the processes' RAM usage apart from using top/htop?

Use vmstat or free command.
There may be some option in sar also for historical memory usage.

What is (used) memory ???. This is a real can of worms - even the kernel devs can't agree on it.
Have a read of this to get you started. Some of the follow-up comments are actually worth reading - lots of noise though as well.