Interesting question and I am glad you brought it to WHT.
It has been awhile since I've hosted many CS1.6 servers as the majority are CSS now, but your arguments sakes CSS is more a showstopper than CS1.6 so it still works out experience wise. My first question is: At what tickrate and FPS will these servers be running at? I think that will have a greater impact on my suggestion that anything else.
The Xeon based system will cost more overall but I am pretty sure the CPUs will outperform the Core2Quads, the reason being:
-The E5430 is a server based processor and its equivalent desktop CPU, the Q9450, is not the CPU you would be receiving if you went the two server route
-The E5430 has double the L2 Cache as the Q9300 (12MB per CPU for the E5430 vs. 6MB per CPU for the Q9300)
-The E5430 operates at a slightly faster clock speed. Gong with the Dual Xeon setup would leave you with 1.328GHz more overall speed spread across your 8 cores.
E5430 Dual CPU - 7,684
Q9300 - 3,348 (3,348*2= 6,696)
I currently have a Q9400 based server and it does not seem to handle the same way an older server I used to have when it comes to CS and CSS game servers. The old server was a X3220 (2.4GHz 4MB L2 Cache, 1066FSB)
The benefits of having two servers is that you can balance the load over a greater number of physical machines, better redundancy (as in not all your eggs are in one basket), more HDD space, and more bandwidth.