The answers you seek are in the F.A.Q. of the main website
http://www.planeshift.ithttp://www.planeshift.it/community_faq.html#minhwrequirementsis exactly where your answer is.
As it is stated sadly your computer will not run the game.
Though the hardware stated is about a 600 MHz game most modern games of similar style are recommending 1 GHz and a 32 meg 3d accelerator, I anticipate this will probably be the ball park if the final product, though I wouldn?t be surprised if it would be more like a 1.6 in the end.
Refining the game to be as processor light as possible can be a royal pain and making it older video cards can be an even bigger pain.
Though I realize that many people would rather have a game that plays on older computers, when designing you want your game to last, not be outdated when it comes out, as such you build to bleeding edge when you design and by the time it comes out it is on par with 50% of the gaming computers on the market.
Rowman not to seem smug but there are CPU?s that have more then 10 times the processing power of yours, this is going to be a slick looking 3D environment and if it was built to run on an older machine, well it would be about at the level of quake 1 or Heavy Gear 1, this is going to look allot nicer.
But fear not, you will have time to upgrade as the game is not even in the Alpha stage of development, as such 6+ months, it is probably a good time to start saving money for a good machine now, you would be surprised what $600 can get you specially if you don?t include a monitor.