What's a good Player Slot / RAM ratio?

I'm starting up a dedicated server, and I'm wondering how many players I should be able to fit on my server without it starting to get sluggish.

Currently I have 4gb allocated, and the server has 8gb.
The CPU is an AMD Phenom II Quad Core, so there shouldn't be any problems there.

I've seen servers running 100-200 slots, and one that's running 500, but that's getting to be a bit insane.

Any feedback is appreciated.

 

Generally, the ratio are these:
1 GB = 20-25 players
2 GB = 40 -50 players
3 GB = 65 -75 players
4 GB = 90 - 100 players
6 GB = 135 - 145 players
8 GB = 180 -190 players

Note, the ratio depending on your plugin configuration, O.S., and much more.
Also note you must have sufficent bandwith for the players... 100 mbit = 320 players

Hope I helped.

 

not to seem antagonistic about CappyT's statement, but I'm guessing you will be running a server with a few plugins. Granted, you will also want to have as little lag as possible.So, running a few plugins that add administrator capabilities and world monitoring plugins (think WorldEdit and BigBrother) along with something like permissions or GroupManager to allow certain people to do certain things, and you're looking at a reduction in numbers of people.
So, for a ballpark estimate, 10-12 people per 1GB of RAM on your server should make good gameplay
Also, make sure your upload speed is more than 1Mb/s (100KB/s) and you should have a server that can work just fine.

 

Thanks for the recommendations.
I'm on a 100mbit connection, and I've got some plugins, mostly low profile ones.
I'll probably go with 100 players and see how it goes from there.
I can always up the ram if it ends up being a problem.