Then you will be able to upgrade to a Core 2 Quad.
Dell hasn't really stated if there are any particular ones that are supported so im assuming all except the extreme editions are supported so i would recommend getting a Core 2 Quad Q9450 or Q9550 since they are pretty common on ebay.
I would also recommend upgrading to the A12 BIOS revision (heres the link - http://www.dell.com/support/home/us/en/19/Drivers/DriversDetails?driverId=R250485) if you haven't already.
Also don't forget about cleaning and applying new thermal paste, this will help cool your CPU.
I'm not sure what TDP your heatsink is rated for but it should be fine if not a bit noisy under load so be aware of that.
If CPU temperatures do become high then report back to us.