That is a great system. Not sure about that specific game. What about performance in other newer games? Do you have problems as well?
pillar is correct in saying to make sure you are using the latest nvidia driver. However, given a gtx 570 is a bit better than the recommended 560, you should easily be able to play on low settings without a problem. Given you have more than the recommended, I would say you should be able to play on high assuming you are playing at 1920x1200 or less.
A quick google search brought up some issues others are having with decent systems and they conclude that it is the game itself. Have you consistently gotten bad fps or is it intermittent and has it been like this since you first started playing the game?
http://forums.steampowered.com/forums/showthread.php?t=2780884
They seem to narrow it down to possible patching of the game possibly and some said simply restarting the system fixed the problem. A troubleshooting thread exists on the steam forum:
http://forums.steampowered.com/forums/showthread.php?t=2777685
Not sure if you have checked it out. They suggest
Problem #20 Tribes Ascend FPS Fix – Tribes Ascend Lags, freezes, or has poor FPS performance
Try these possible solutions:
Usually you can prevent Tribes Ascend from lagging or freezing by disabling vsync and antialiasing on the Tribes Ascend’s exe file via the AMD Vision Engine Control Center (AMD/ATI) or Nvidia Control Panel (nVidia)
Alternatively you can stop Tribes Ascend from lagging via your PC task manager. In Windows 7, press CTRL + ALT + DELETE and click on Task Manager, go to Processes, then right-click on Tribes Ascend’s respective exe file and set priority above normal or higher. You must do this every time you play the game.
In addition, your anti-virus may be the problem here. There are certain anti-virus software that are not ideal for gaming so, check our Technical References page for some suggestions.
If you’re still using Windows XP, upgrade to Windows 7. Honestly, a lot of users have attested this to be the fastest solution they’ve tried.
For more advanced users, try enabling/forcing tripple buffering and V-sync using the D3DOverrider software. Check our Technical References page to download D3DOverrirder / RivaTuner.
Finally make sure that your Internet connection is stable. This is a commonly overlooked problem especially for online modes / multiplayer games.
If the above still do not cut it out for you, check out our tips on reducing lag.