I read and heard that even though there is no official requirement 512 Mo of memory at least is needed to play Planeshift.
I use
a P4 2,4 Ghz processor
only 256 Mo of Rambus 800 Mhz
WinXP Pro
and it runs just fine !
though i would not recommend this type of memory (expensive, need to be paired, overall performances ok ... so what)