If your building a dedicated server then a dual core 3gig P4 would do nicely, but the E6300 would be better from the heat/power perspective and would be faster - tho that would only matter if you were running 5+ from the same box...
as the previous poster said - fast RAM doesn't really matter - but having quite a bit of it does... I wouldn't go for the cheapest board however - you want something thats going to be reliable in the long run...
I'm not sure off hand what server case you have and so what will fit into it, a 1u case isn't easy to fit stuff into...
not sure of your requirements for PSU (need something designed for 1u case) and hdds, etc... but...
RAM
===
heres the ram for multiple servers:
http://www.overclockers.co.uk/showproduct.php?prodid=MY-010-OK
if you are just running 1 maybe 2 servers off the box then you could go with:
http://www.overclockers.co.uk/showproduct.php?prodid=MY-011-OK
if you were running windows or if you really wanted to keep costs as low as possible and were running a linux OS such as debian then you could run atleast 2 servers fine + the OS with:
http://www.overclockers.co.uk/showproduct.php?prodid=MY-080-CR
CPU
===
http://www.overclockers.co.uk/showproduct.php?prodid=CP-126-IN the E6300
however if you really want to keep costs low and are running 5 or less servers on the box then one of these is more than upto the job:
http://www.overclockers.co.uk/showproduct.php?prodid=CP-057-IN
MOTHERBOARD
===========
Not an easy one as I don't know which will fit your case... normally a 1U will take almost any ATX and micro ATX board tho.
http://www.overclockers.co.uk/showproduct.php?prodid=MB-120-AB
something cheap that should be reliable... or if you want to spend more money and probably get a bit better performance...
http://www.overclockers.co.uk/showproduct.php?prodid=MB-011-IN
I can't make any guarantees as to the compatibility of any of those items in conjunction with each other...
I'm sure your aware of bandwidth, etc. but just in case...
a full 14 player CS:S server is going to be chugging out a good 1-1.2Mbit of outgoing bandwidth and receiving 500-600Kbit... you could get away with less but it would start to impact on the quality of the gameplay.
Also a 14 player CSS server that was moderatly active is going to get through atleast 100gigs a month, if its really active it could be 200gigs.
EDIT: A gigabit NIC is a good idea, even if you were only renting say a 5mbit pipe as most DC/NOCs use gigabit for their internal network even tho they are probably fully switched and capable of running devices at 10-1000Mbit. Also watch out for the PSU requirements - if the 1U case comes with one they are often only 180-250watt which would put a limit on your hardware.