Secret Socks :
I'm sort of a noob with this kind of stuff and I have a spare E8400 cpu lying around. Will it work as an upgrade for my old Dell XPS 420 with the LGA755 socket?
First, it's socket 775, not 755. Then, yes it will work, but the E8400 is less good than the core2quad inside your Dell XPS 420 (assuming you haven't downgraded it since you bought it)