Wireless (801.11b) is also HalfDuplex and 11Mbps is if you are close to a AP. Wireless is also open to interferance that can cause occasionial Packet loss (usually around 1-2% at worst on a good connection). At farther ranges wireless can drop to 1Mbps.
Wireless isnt great for game servers ...mainly based on what I stated above. As far as cable or DSL they are tolerable for small groups but tend to be unstable once the numbers get much over 5-8 people. They also are affected by (on cable) shared bandwidth problems that can cause inconsitant network latency based on time of day.
DSL can be good if you have more than 256k Up traffic. 256k is good for about 4-7 people depending on the game and server settings. a 768kx768k circuit would be a good one for a 10 person game server. I am basing this info off NWN,Quake3, Halflife (and mods),and UT net code. UT2003 seems to be similar in usage but havent had a large amount of time to play around with it.
As far as hardware a 128meg PII233 can run most game servers out there if its running Linux or NT4. You will need a bit more power if you are running Win2000 or XP espically in the memory area.Just make sure you have a full duplex PCI NIC and a unload PCI bus (ie not having all your PCI slots full with hungry devices) or even better Multiple PCI buses (ie serverworks chipset) A switch also helps if you are a in a busy network envroment.
If you have any questions please feel free to email me and I would be glad to help you in setting up a server to run on Cable or DSL.