I had 2 nic cards installed on my pc with just a crossover cable to simplify the setup to the xbox,but now I have a d-link router hooked up to get the firewall protection(plus eventually hooking up my ps2). Can I still just use the crossover cable to the extra nic card or will I have set it up through the router with a straight cable?

crossover cables are for pc to pc only

if you use a router you'll have to use regular network cables

I guess my question is, Is it possible to just leave my xbox hooked up with a crossover cable to the 2nd nic card, while having a router hooked up to my pc,or will there be conflicts with the IP's?

yes it's possible if you set it up correctly. Read the tutorial in my sig, configuration 2.

Cool. Thanks for your help!