DNS
Ihorwinkle makes a great point...
As I move forward improving my internet speed (latest improvement: upgrading my Cable Modem to DOCSIS 3.0 [Moto SB6120, if anyone cares]), ensuring the most responsive DNS Server for your network is the best bet at realizing faster internet browsing, etc. It may not directly improve your download speeds (a more effective DNS Server, that is) but it will allow you to get going where you're going at a faster rate. I shaved 44ms off of my DNS queries by switching DNS Servers.
Since I'm a Google Fanatic ("Fanboy" makes it seem like it's not a choice), I went with Google's DNS Servers...which were/are fast, but Comcast's DNS Servers were just better for me. An easy, definitive program that will figure this out for you is called NameBench. it's freeware, and small. The analysis took my machine about two to three minutes to complete. Just do a Google Search for namebench (one word) and it should be the first selection.
Hope this helps!!
As far as improving download speed, there are a few tweaks you can do, but truly, Microsoft spent a lot of time ensuring Win7 performed well. Look on the brightside: They already tweaked it for you. lol
Minus the graphical performance improvements (which aren't really improvements to me since I enjoy enjoying looking at my GUI--so 21st Century), most other tweaks that you do will take away a convenience. Disable Indexing...you lose the convenience of the search functionality; disable Aero...you lose the snap functionality (which is quite important if you multitask a lot)...
With all that said, I think we all want to believe that we can get more performance than the out of the box experience...ahem--I am here reading through the boards as you are lol Windows XP was not really optimized for the internet so there was much room for improvement... That just isn't the case with Windows 7 (a good thing, for sure). Even looking at the improvements to Window Firewall and the serious customization options of that alone is quite impressive; lightyears beyond WinXP...
I would still love to know if anyone has done anything to improve their network performance...