It's on the Gameing and Applications page under QoS (Quality of Service). If you are using DHCP to assign IP addresses, be sure to setup that addresses under "Static DHCP" so that it doesn't change. If it's plugged into an ethernet port, you can also select QoS by ethernet port.
I've been playing with different incantations of the WRT54G firmware. All of them them have various levels of QoS or bandwidth management. However, I've noticed that DD-WRT has added a WonderShaper feature by which it can filter by application such as "bittorrent" or "voip". Looks useful but I haven't tried it yet.The problems start when you want to restrict some users traffic, but they want to use VoIP, streaming video, or other form of time sensitive content. That can make crude bandwidth management by IP less than useful and rather complex. After reading: decided that I wasn't going to write the ultimate bandwidth management script. Anyway, think about what you really want to control.
For additional Linksys specific help, I suggest the forums at:search for "QoS".