Re: Networking for a LAN
You will be able to run simultaneous games across that one switch.
Personally, if you want the games to run as smoothly as possible, then I'd go out and by a switch which you can configure Vlans on.
The reason I say this is because you don't want Halo broadcasts and GTA4 Broadcasts on the same switch! All game packets that are sent out for GTA4, will be flooded to all ports on the switch..regardless on whats connected to them.
Vlans will allow you to split up your broadcast domains for both games, which in non-geek language, means you can have HALO3 designated 16 ports, and GTA4 designated 8 ports. (Means HALO3 and GTA4 will be on different networks on the same switch). This will require you to manually configure IP addresses on each xbox (but thats pretty simple to do).
Does the switch that you're looking at purchasing have VLAN capability? if it is, I can talk you through the steps for configuring the VLAN. It is not difficult at all.
P.S... You may find that running both games on the same switch is fine without having to use VLANs but as for latency issues, you wont know unless you put the network together and find out. I'd rather be safe than sorry.
If you want some more advice, just flick me a PM, Ill be more than happy to reply.
Depending on the speed of your switch will also be a factor. 100MB out each port should be ample for what you want to do. 10MB switch out each port won't do you any favors.