Answering you question about multiplayer games, to start out, there are only a few I would suggest you getting.
Counterstrike Source
Half Life 2 Deathmatch
Counterstrike 1.6
Day of Defeat Source
Unreal Tournament 2k4
Call of Duty 2
Warhammer 40k: Dawn of War
Company of Heroes
I think those should keep people stuck to your cafe, and none of those require particularly demanding computers at medium to low settings.
And have some non computer attractions to rest peoples eyes, such as Air Hockey, Foosball, and PingPong. This way, it will be known as a place where kids can come and hang out after school, although I do not know if that is the reputation you really want.
There are a lot more suggestions, but here are the first few that I am going to lay down as groundwork that ANY cyber cafe needs.
20 computers, medium specs. I would suggest Athlon 64 3200 processors, Nvidia 6600GT vid cards, 1gb of ram, and everything else as cheap as you can be. No speakers needed, just have a headset next to each computer.
Wifi for laptops and portable devices, fast hard connections for desktops.
I hope that I helped, and if you need any more advice, IM me on AIM at dexcarm89, and I can help you out there. I guess I just have a good mind for business, and I am your target demographic, so I figure I can help.
Dex
Attractions besides