You bought a working machine and you want to mod it.
Ok to that.
You want to use it as a mail server.. sounds good. Alternatives are firewall, dedicated CD burner, word processor..
But the question about an alternative mobo is kinda throwing me.. why did you buy it in that case? I'd say considering you paid money for it I'd put it to some use, you must be able to get drivers for the NIC or replace it.
If not I'm pretty sure I've got one of those lying around work somewhere, I can always do a hunt for the drivers so you can get it on a network / the internet, not sure what OS is actually on the Dells I've got but there is a box marked "Dell stuff" taking up shelf space that I can go through if you want to see what is in it.
Chances are the drivers are on the HD somewhere in a non-standard folder or you could stick a higher end OS on it that will auto detect the network card.
Finally check
www.bootdisk.com - there are drivers for nearly anything on there under network if memory serves.