I'll second Tribes 2. I'd say a good system was more important then a good internet connection. I play on a 56k, and it's decent, just as long as my family stays off the network.
With your Geforce, Tribes 2 should run great. The Voodoo owners had problems, but Sierra has since toned down the graphical quality in the game to accomadate them.
If you want to see what it looked like before the VooDoo patches, load up Single Player/Lan mode and host a lan game. Don't get too attached to it though, cause as soon as you go online, it'll patch itself to the latest version, and the fog will appear.