what cpu do you have now? the q6600? not sure if it came with any other cpu's or not.
i run one with the q6600 myself
edit: dell page says the following cpu's are supported
Processor Type
Intel® Core™2 Duo and Core™2 Extreme processors up to 130W
Families
Conroe, Kentsfield, Wolfdale and Yorkfield
would have to look and see which cpu's these are but even the q9*** series is not much of a step up over the q6600 and would not be worth the cost.