Mac Mini - Troubleshooting wifi signal

After replacing my Mini's hard drive, the airport signal strength is averaging one fewer bars. It shows a single bar more often as well and the number of bars fluctuate more. I triple checked when I put the antenna back, it was back exactly as it originally was. The shielding is near where the wire connects to the Airport. Perhaps I did something to the antenna I'm unaware of. I did notice that the Airport had two antenna connectors and I'm using only but I'm pretty sure that was the way it was before. My Mini is really old - it's a 2006 model - Intel Core Duo, as is the OS, 10.4, Tiger.

Perhaps I could replace the wifi antenna?

Will something like this boost my signal?