i want to buy a new dual core processor (amd atlhon 64 4200+) i'd like to know if fluent consider that as a mono-processor unit or i can choose to use the parallel version of fluent

yes, fluent can make use of the second processor. You need to run Fluent parallel to use both processors.

Actually that may not be necessary for dual core systems. For SMP systems however, where you have 2 physical processors on the same motherboard, fluent parallel will be more efficient than fluent serial.

ok, correction. I just checked. Fluent parallel gives me better performance on a dual core than fluent serial. I used the 'smpi' and -t2 switches. This can also be done in the GUI (File -> Run). Remember not to start with the solver (2d, 3ddp etc.) already specified.