When to Increase the amount of Physical Memory?
Generally you should upgrade your memory when you have less than around 300 MB available. This minimum amount is needed for the disk cache to be effective and for Windows to be able to satisfy temporary memory requests.
At what point will memory usage negatively affect game server performance, assuming a CPU load below 50%?
The two are mostly independent.
Is there any performance impact on a game server when using a dedicated IP versus sharing an IP and using different ports?
How many ports can be used for game servers on a VDS? If using multiple ports, does it matter if the port numbers are in sequential order (i.e. 27015, 27016, 27017, etc.) or not?
You can use as many ports as you'd like, but keep in mind that our default DDoS mitigation rules only cover common ones, like 27015 to 27030. If you use multiple ports, just make sure that your servers don't step on each other, because some games (like Orangebox) use multiple ports per instance. Check which ports each instance is using with "netstat -ano" output.
My three L4D2 servers all run at 400+ FPS on Windows 2003 Server. Given that L4D2 is somewhat unique, can the FPS be further increased with Windows 2008 and HPET as with other Source games?
Yes, I would expect it to be able to be increased to 1000 under Windows 2008, as with other Source-based games that use the older tickrate/FPS model. However, L4D2 has such a low and locked tickrate (30) that the FPS doesn't matter very much; you may actually have better results (lower CPU usage, for instance) from running at a lower one.