I'm pretty sure it's soldered onto the main board meaning you'll probably break the iPod by trying to even remove the flash card that's already in it. Plus even opening the iPod will instantly void the warranty plus, if it stops working further down the road, Apple will refuse to help. No iPod has parts that are intended for the user to change, and as a result Apple refuses to deal with people that decide to do that.
So basically, you're much better off just buying it and using it how it is - you're almost certainly going to break something while trying to switch it, and even if you don't you won't be covered by any sort of warranty.