How much ram does a bukkit server need?

I have an entire written up synopsis of specs needed for perfect performance on servers:

http://www.minecraftforum.net/topic/120 ... /#holdings

That being said, when renting a server online, usually network speeds are no longer a factor, removing a potential bottleneck. As for other things you need to concern yourself with is the power behind the cpu vs your ram. A minecraft server really doesn't need much more than 8gb at maximum if optimized properly with a moderate amount of players. The main bottleneck you want to check is the CPU. Typically speaking, you're looking for the quality of the cores themselves over amount of cores. To summarize, clock speed + IPC of the cpu is more important than it being a quad-core, as minecraft does not manage much multi-threaded tasking.

So, to answer the ram question (when hosted elsewhere), 8gb is about the most you'll need.

it's not a possible question to answer because most of the time RAM isn't a bottleneck factor to the amount of players you can hold. A lot of the time, it comes down to your CPU capability and your network speeds (if homehosted).