So I currently have an HP G5210uk -
and I want to upgrade my processor, but my motherboard a M2N68-LA (Narra6) only supports processors with a tdp of 95w. So I 'm thinking of just buying a new case and motherboard along with a processor, but I'm not sure if I need to buy a new operating system since I will be using the same hard drive.
Due to the fact that you'll be changing the graphics, processor and motherboard signatures, your Windows version will try to re-authenticate itself (if it manages to boot!), and will probably fail to do so: since you have an OEM version of Windows (it'll be looking for an HP signed BIOS), it'll tell you that you need to buy it anew - and at full price.
You can try calling Microsoft, you might get a discount, if you're really lucky; more likely, they'll redirect you to HP, who will tell you "your system's configuration isn't supported by HP" and that'll be all.
If you are having issues with the operation of your motherboard, you may replace it --- but you cannot use the mobo replacement as a *slide-ways* upgrade. Essentially, the replacement mobo component should be of similar chipset as the original.
In your case, that would be an nVidia MCP61 chipset.