I have just upgraded my pc from athlon 800mhz to 1.4ghz athlon and also the memory from 384 to 640 of ram to cope with large file sizes. an original render of mine took 26hours i have just done it again with the 1.4ghz and wow !! it only took 16hours thats 10hours less this is with b4 so i would say upgrade the processer if render time is a pain.