How to connect 2 pc in a router to 4 internet

I have a DI-604 - Dlink router and a 2mbps broadband connection..when im using my 1st pc to access the internet the connection is fine but when i try to connect my 2nd pc at the same time my internet speed slows dramatically like it take 2-3 minutes to open google and everything. but when i try to access the internet one pc at a time it works fine with any of the 2 pc's that i have. do i need to configure anything in my router? if so can you please provide my a step by step on how to do it because im new at this... also i need to have my 2 pc connected locally so we can play LAN games.. thank you in advance...