Stop All The Downloading
Estimated Cost: $0
It's pointless to spend all this time tweaking and improving your system, just to have your internet connection causing incredible lag when you're playing online.
This may be the simplest, and most obvious tip to follow, but you'd be amazed how many folks head into Team Fortress 2 with a few terabytes of pornography on the download.
Make sure you shut down any torrenting programs and pause any browser downloads before heading online. Your teammates will thank you when you're not darting around the map with wild abandon, and you'll feel better knowing you're the only thing to blame for your failures.