I played happily with GeForce 8800 without lowering any default settings. Of course, 8800 is the top end of its generation and 9300 is the bottom [of the next one]... but I'm not sure it makes it so much worse. Should be ok, I guess.
...Oops, I forgot that my 8800 has 768 MB of video memory
Well, its playing ok on low settings @ 1600x900
Win 7 x64 ultimate.
What would be handy - as this is where I am currently experiencing slow down - would be a specific toggle to reduce the quality of the teleport fields. IE the puzzle areas on level 4, there the number of fields make the game run slowly.