http://www.wowinterface.com/downloads/info13581-LeatrixLatencyFix.html
Yes. I know. There's a WoW logo thingie there. Ignore it.
This registry fix really, really helped my latency issues.
My Guild Wars ping used to be 600-900, now it's 300-400. Oh yes, and it fixed my WoW ping too, (even if I'm not playing that anymore).
I'm pretty sure it helps / will help with any other online games I poke.
Best of all - if you don't like it? Comes with uninstall script, and is most certifiably not malware, spyware, or any other evilware.
If you use Windows, and play online games where latency is an issue, get eet. Get eet nao.