Ping makes a huge difference. It's the time (in milliseconds) it takes the packets of data to make it from your PC, to the server, and back again. The higher the ping the further out of sync you'll be with what's going on. For example, with a ping of 200 you'll have to aim well in front of the person you're trying to hit in order to compensate for the latency. With a ping of 30 you can pretty much just point at them and click. Also, if you've got a bad ping there's a good chance the connection in general won't be any good - if you're getting dropped packets that can cause nasty glitches.....maybe you picked up a gun or some health but it didn't even register. Not good. Most games come with some sort of graph you can use to monitor the state of the connection. There are a few tweaks you can do to improve things, but basically try and pick a server that's closest to you (it should say where the server is based) because the closer you are to the server the better things will be, usually.
As a general rule, and depending on the type of game you're playing (shooters are more reliant on a good ping than an RPG) I would say a ping of....
200+ unplayable
150-200 - 56k quality, not good but maybe OK depending on the game.
100-150 - Poor, but I played EQ2 with this kind of ping and it was playable
50-100 - good. Should be the minimum for shooters like Quake, BF2, etc.
20-50 - excellent. This is what you should be getting on a good, local, server
Hope that helps
