Vast Packet Drops: What's the culprit?

Hello. I have a first-gen Macbook Pro (A1151). Other than replacing a hard drive and disconnecting the left speaker (for some reason it was hissing and distorting, even when I replaced the speaker), I haven't had to do anything to it to keep it running. However, I finally have a problem that I can't put up with any longer. My wifi is dropping packets crazy enough that my 25 meg connection is closer to 1.5. I know it's not due to interference since I have another laptop that works just fine. Pinging my router from both laptops from the same location results in 0% packet loss from my other laptop, and 58% packet loss from my mac.

I can easily replace the wifi card. However, I feel like if that's the culprit the OS would be complaining a little more. That makes me lean toward the antenna, but my signal strength seems okay (although it's only a one bedroom apartment). How do I decide which it is? I don't want to shell out $90 for the card only to find out it's the antenna. Can I even replace the antenna?

Thanks for your help.