sumpothed

Hello. here's the situation. I am using comcast cable internet at about 4mb. It goes through a d-link cable modem into an airlink101 router. two computers are connected to the router. My problem is that whenever the other computer is doing anything (downloads and such) my runescape laggggs bad. I mean like 10-15 seconds between when i click and when something happens. Makes for terrible game play. I did an online bandwidth test at Cnet. When i am the only one doing anything, it reports around 4200Kbps. When the other computer is downloading (even when the downloads are capped) the test reports 800Kbps. It seems to me that 800Kbps should be plenty of bandwidth to play runescape, but every time my BW drops below 2500Kbps the game laggs bad. Is there anything I can do to prevent Lag? Is there a setting in the router or modem that I should alter in some way? Could it be my computer? (I am running an athlon 2400XP with 256MB ram). Could it be my Java version?. I have tried world switching to no avail. I dont know what else to do. Is there anything I can do? Please help.