I don't see what's wrong with 100Mbps. It won't affect your internet speed.
If your other computer(s) on your network have 1000Mbps NIC's, and your router/hubs/switches all support 1000Mbps, and you transfer a lot of big files to other computers regularly, and want to save some time, then you should get a board with a 1000Mbps NIC.
Otherwise, 100Mbps is fine.
TheMajorMMX - Intel P200 MMX @ 225Mhz - V-Tech (PcPartner) Baby-AT mb w/ Intel Triton TX chipset - Nvidia Riva TNT2 M64 w/ 16MB - 256MB M-Tec SDRAM - Still running.....