As Karyuu stated, sound is the problem, however turning it off through setup can be a bad move if you run the game in windowed mode (risk of crashing when minimizing or toggling window) untill a bugfixing update is released.
Instead of turning sound off with Planeshift Setup, go in game normally.
Select Options (second button on the top, starting from the left) then Sound-General. Disable all sounds (background, effects, etc.) by unsticking the round boxes.
Despite you had a good amount of memory (with 512 MB the game should run fine) fact is that certain places are overcrowded with sound effects that may have caused an insanely high memory consumption. That place is commonly the arena. Yes, the sound eating memory bug has been looked upon, and, afaik, it\'s performance has been improved, but then again, you\'ll have to wait till the bugfixing update before enjoying the game sound effects without causing your computer to freeze.
Good luck with it.