When faced with buying a 2Gig HBA (Host Bus Adapter) like a fiber adapter or 4Gig host bus adapter you might consider buying the 4Gig HBA. Even when hooked into a 2Gig fiber switch and a 2Gig disk you will get more throughput with the 4Gig adapter. The processor on the 4Gig adapter is better made for higher bandwidths and is usually backwards compatable to 2Gig. It's a lot more expensive for a 4G compared to a 2G but at least you can use it if you upgrade to full 4G network. I've heard this also goes for 100/Mbit vs 1000Mbit network cards. Buy the 1000Mbit cards if you are going to use them on the 100Mbit network. You get better speeds because the card was made to process packets at a higher rate. You might not see much of an increase but you can be sure that network will be running at it's fastest.
Which reminds me of a question someone asked me sometime back. It was: What are the best network cards to buy for servers. Without a doubt it's the Intel server based network cards. They are known as "Intel Pro 1000" series right now. Make sure it's the server based ones not the desktop ones. The drivers cover tons of os's and were written by Intel themselves. The driver supports checksum offloading, jumbo frames, and many other great things. The BSD and Linux drivers are fantastic.