but any answer would be relative to the purpose that needs to be served. All of these are "end of life" products but you can probably drop the dual core and make lowest price the determining factor between the other two. Either of those might make reasonable Linux machines.
Which on of these is better?
Intel Core 2 Quad Q9300 CPU/SLAWE/Socket 775/6M/FSB 1333/2.5Ghz/45nm
Intel Core 2 Quad Q8400 CPU/SLGT6 R0/Socket 775/4MB/45nm/95W 2.66ghz
Intel Core 2 Duo E8600 3.33 GHz Dual-Core Processor