Inspiron 546 slow internet connection

I have an Inspiron 546 and am using the build-in NIC. I recently upgraded my cable modem and router to handle my new ISP's download speed of 100 Mbps. While I can consistently achieve this speed with the  Optiplex 3010 that's also cabled to my router, the 546, also cabled to the router, can at best get about 50 Mbps even when optimized by using TCPOptimizer. All cables are Cat 5e.

I'm wondering whether a better NIC would improve the internet performance of the 546, and if so, what NIC should I get.

Thanks,

Tim