Any money you spend on that will really be wasted. It's so old I can't even tell if it's Socket 478 or LGA 775. It has an Intel 865 chipset and they were available with both. If you can go to a PC shop they may very well have an old stock cooler that would fit laying around for cheap or even for free. You will need to know your socket type though.
Socket 478 look like this:http://en.wikipedia.org/wiki/Socket_478
LGA 775 looks like this:http://en.wikipedia.org/wiki/LGA_775
You can also try cleaning off the cooler you have with a can of compressed air. That will be the cheapest, easiest option. If you do get another cooler don't forget you will need new thermal paste as well.