CS, DoD, and CS:S are by far the most popular games. Perhaps second is BF1942, and Desert Combat(a mod for it)
CS, DoD, and CS:S all require very little space, in comparison with other game engines. A duel Opteron 242 with 2 Gigs of ddr400 memory can hold a maximum of around ~192 users, and costs are around 2-3$ per user. Griffinrun and krypt charge 3-5$. I've determined not to host more than 128 users per server, and not more than 64 users per athlon 64 3200+. The reason I've opted for AMD processors, and definitly 64-bit, is because Valve offers a 64-bit optimized version of hlds.exe Combined with Fedora(because i cant afford Red Hat enterprise), you can fit quite a few people per server. Measuring bandwidth, each player required 4-6 kbps, depending on the mods (adminmod, amxx, amx, statsme, warcraft3, SuperHuman, etc.).
If you want to provide normal services, web hosting, voice hosting (Ventrilo and/or TeamSpeak) is required. Most also want ftp access, for maps, and textures and such.
If public servers are your focus (they cost 30-100% more than private, passworded servers), then unmetered bandwidth from your backbone is a good idea. Private servers are empty more often than not. But either way, during CAL season, some nights are very busy on private servers, and servers cannot be ever close to overloaded on normal times.