I have a wireless DSL modem, to which my laptops (Macbook, and Macbook Pro) connect just fine. My Tower also connects at full bars, but it runs about 25% speedwise. I'll run a speed test and get 300Kb/sec on the tower, and 1200+ Kb/s on the laptops.
After upgrading the tower to Leopard, the problem still exists. Everything has been rebooted, etc. Any ideas?