I'm building a gaming rig and my motherboard will be the DFI LP LT X48-T2R. Will I gain any noticable latency performance using a network card over just the onboard LAN?

Overall, no.

don't waste your money

Not a standard one no, you'd have to spend on one of the ones with onboard ram like Killer NIC but only get that if you absoulutely want to pay £50 for like 10 more fps / 10 lower ping in games.

Thanks for the advice, doesn't look like I'll be needing a network card.

Well hold on a second here.
If you really have the money to blow and play in the competive gaming scene then yes it may be worth it to get a good quality network card.
Not some cheap 10$ one though. It would be somthing like spanner_razor mentioned, like that killer nic. Its expensive though, and you will probably see a small difference.