Onlinegaming is a bit special. It is important to have a good rate of both up and download-speed. Since you are playing on big maps, there is alot of information that will be passing between you and the server. When there is not sufficent rate of information being traded, then you have that well known "lag".
For example: I have 2Mbit up and 2Mbit down. Lower then this would probably get you in trouble on 64 player maps. I get a little "lag" sometimes, but it helps if i turn my firewall off.
My specs.
Processor: Pentium4 650, 3.4 GHz
Hard drive: 200 GB SATA
Video card: XFX GeForce 7800GT 256Mb PCI-Express.
RAM: 1 GB