I need another slot for putting games on my xbox. Can I just get an ethernet card?

I'm uncertin what your asking. I'll try to answer but you question.

So if your asking about adding another ethernet port to your PC, yes(depending on the machine). As long as you have a open PCI, PCI express, ISA, USB, or PCMCIA port then it's just a matter of getting a ethernet card(or dongle) that matches the open port you have(asumming you can locate drivers for your OS).
If you plan on connecting without a hub, switch, or soho router/switch, then you need a crossover cable to connect the pc to the XBox.

On the same note as the last, if you have a hub, switch, or soho router/switch your pc is currently connected to and it has open space then you should be able to connect the XBox to your hub, switch, or soho router/switch. Normaly you can add another IP to an card incase you want to keep two logical networks.(note: you may need to static your pc's internet connection IP)

Ok if your asking about adding another ethernet port the XBox, at this time I do not know of a way to do that since it lacks a pci(or other) interface.

Yeah, I want another on my computer. I do have acouple PCI slots so I will get an ethernet card. Thanks, I just wanted to clarify that.