This is pretty well known stuff and doesn't make a huge difference, but it helped me and I couldn't find it after searching here so I thought I'd share it with you guys.
Disclaimer: I honestly am not 100% sure this works. Several of my friends did it, and I did it, and it seems to have made a difference, but none of us actually recorded our latencies/DL speeds before hand.
Basically, no one program can use one-hundred percent of your bandwidth, they max out at eighty. The other twenty-percent is "reserved" for other programs with a higher system priority in case you want to run more than one at a time. This guide will allow WoW or program you'd like to run to use one-hundred percent of you're bandwidth even if a program with a higher priority to the computer is requesting access.
First of all, if you're using XP Home edition you don't have the Group Policy Editor which is used in this guide. To install this, please download this .rar file. Inside you'll find all the files needed to install and instructions on how to do it.
After that's done or if you have any other edition of XP than home, here's what you do:
1. Click Start, then Run. Type "gpedit.msc" (without the quotations) and press enter.
2. Click through: Local Computer Policy, Computer Configuration, Administrative Templates, Network, QOS Packet Scheduler, Limit Reservable Bandwidth.
3. Double click on "Limit Reservable Bandwidth", enable it, and set it to 0.
There you go. WoW can now use 100% of your bandwith.