I seriously doubt that it will make any difference in latency. Other than large file transfers over your LAN, 100Mbps wired is very likely faster than your Internet service plan so it would not speed up downloads or anything like that.
Test your latency from your current card at http://pingtest.net to see if your current connection has excessive latency.
I don't really see much benefit other than a bunch of software stuff that it has available. My slowest NIC (out of the 8 computers I have with 100Mbps or gigabit adapters) pings my gateway in under 1ms; and 18ms to the Google DNS servers (220.127.116.11).
For hard core gamers it improves UDP packet transmission, bypasses the Windows network stack, and prioritizes gaming connections ahead of other stuff, average guys like me would not notice much difference, but if you are primarily a gamer you would notice some, although exactly how much would depend on lots of factors.
The actual numbers that I have seen in reviews, indicate a slightly less than 5% improvement in gaming latency. If you have already spent enough to max out your video, then maybe it's worth it to you.