It would be socket compatible, however you'd need to check with Dell to ensure that the Q6600 was supported in BIOS. If it's not, the future upgrade won't work.
Another option you could consider (if you already have a hard drive, DVD drive, PCI-e video card and a copy of windows) is a Q6600 barebones kit.
* Black Mid-tower case
* 600W Ultra PSU
* XFX Motherboard
* Q6600 CPU
* 2 GB Crucial DDR2
The kit is currently on sale for $299.99 at Tiger Direct
all you would need to do is plug in your hard drive, DVD drive and video card, install windows and you're good to go. (If your budget is tight and you don't have a PCI-e video card, you could always use the onboard video until you can get a real video card)