You should be OK to upgrade since your desktop has other CPUs as optional configs:
The remarks about the BIOS being able to handle the CPU are valid, but I think it won't go crazy given the above. You might have to update it, though.
Check this Newegg page
(no, no commission for me). It lists all options for LGA775.
Now, check this post
from Monk, a lot of wise advices. You should not spend 200 bucks on a quad core if you are going to work on spreadsheets or collect pictures and organize them with Picasa.
Good luck, let us know if you need advice replacing the CPU. You should get a compatible fan (if not coming with it) and you should apply the thermal paste correctly.