Ah, an interesting upgrade. Does the BIOS and chipset support this chip? Perhaps with limited capabilities? Sometimes these Dells use a 5-pin fan connector that I have seen, but perhaps something that old used a 4 or 3-pin connector that would be compatible.
I agree with the re-apply thermal paste suggestion after a good cleaning.
The beginning of wisdom is this: Get wisdom. Though it cost all you have, get understanding. - Proverbs 4:7