Maybe this will help.
It comes from here:
http://planethalflife.gamespy.com/View.php?view=HL2Guides.Detail&id=4&game=3
It's a very nice guide
Servers use a lot of bandwidth (internet connection speed), memory and CPU/processor power. Because of this, not everyone can run a server; the most common bottleneck is the internet connection (usually isn’t fast enough). I will list a few things that are important for hosting a server—you should have at least the minimum requirements listed below—if you don’t there is no guarantee (and a high chance) that the server will not run optimally.
Today’s computers are capable of running most servers, but you should always take into consideration just what you want to host and to what level. There are limitations on everything and some machines might not even be able to run the server at all.
Processor/CPU power: To run a Source dedicated server you need at least a 1.0Ghz Intel CPU or equivalent. This will run a basic server with 20 slots at a good level of performance.
Memory: To run a normal server you will need at least 512MB RAM, you might need to restart the server every once in a while to clean up the memory. 1GB is enough to run over 60 player slots total.
Bandwidth: Bandwidth is used a lot—more specifically, upload bandwidth. Here is a list of how fast your connection must be to run a certain number of players at lowest settings. (When talking about bandwidth for servers we always talk about upload, not download.) Note: these numbers are theoretical, your results may vary slightly!
Upload Speed (Kbps) Player Limit
240 6
320 8
400 10
480 12
560 14
640 16
720 18
800 20
880 22
960 24
1120 28
1280 32
1440 36
1600 40