Why do I only get half the speed I should get with my DSL?

-I have 6Mbps AT&T and my fastest download speed I have ever got was 350kb/s shouldnt it be more like 600kb/s. Oh and if this helps I have 2 computers a brand new core i5 sandy bridge laptop and an older computer with a pentium 4 desktop. I mostly use the laptop on wireless and the desktop wired and I get the same speed on both. I am getting mad that I upgraded to the fastest internet connection from one of the slowest and my speed hasnt increased that much. I had 1.5Mbps and I get about the same speed since I switched