Agreed with setishock. The problem with hubs is that due to the way they work (sending all the data everywhere) you get loads of collisions between packets, which mean that those packets are destroyed and have to be resent. Now I'm using a hub on an old, low traffic network of 3 PCs that's non critical. That sort of use is absolutely fine and not going to cause any issues at all really. Definitely nothing noticeable.
However, whack many more PCs on there with higher traffic and you'll really notice things start to slow down and become more unreliable. A decent switch effectively eliminates collisions so solves this problem, and switches these days are pretty much the same price as hubs. I'd seriously consider investing in a switch, you will notice the difference if you're using more than 3 PCs with moderate to heavy network usage!