I have never seen someone overclock the CPU on a Dell. They lock certain parts of the BIOS to prevent the average user from f-ing up their computer. You can overclock your graphics, however. I do it on my Dimension 8400. Out of curiosity, what chipset is your mobo? you may be able to do better than the D945. If you don't know, download CPU-z off of cpuid.com to find out.
I would recommend that you do a Google search before you give up hope about overclocking.
Yes, it means that you chipset is an Intel 945 series. This could be good or bad. It may be good because it may mean you can put a Core2Duo in your machine and thus illiminate the need to overclock. It could be bad because it may also not work. If you want to take a chance, go to newegg.com and buy a Core2 and see if it works. If it does, return the D945. If it doesn't, then just return the Core2 for a refund.