ethan - one important question is what is the slowest link in your download process? Find that and that will be the place to start. For example, adding additional memory and / or a faster hard drive will not help download speeds if you are on a dial-up modem. Upgrading to a faster DSL or cable modem speed will help in that area. What software / applications are you trying speed up? Some are cpu intensive others are gpu intensive.
I have wasted some $$$ in the past chasing a performance increase without first understanding what was involved.