Chances are the card is 2.4 GHz only, given the age of the system.  And while you can upgrade the card (you'll need the old half-minicard PCIe type, not the current M.2/NGFF format), before buying check the condition of the antenna wires inside the system.  If they're loose from the card or there are breaks in the insulation (carefully check in particular the area where they pass through the hinge), that can cause intermittent connections. If the antenna wires are damaged, you're better off with an external USB wifi adapter.  If they're intact, you can choose from whatever upgrade card you'd like.  
I have Vostro 1550 and installed Windows10 recently. Since then I can connest Wi-Fi only sometime. Then I`m thinking about changing from 2.4GHz to 5GHz Wi-Fi. Do I need to replace Wi-Fi card ? In my PC I found the driver ...802.11n.. Does anybody know how to configure if the existing card can cover also 5GHz ?
Vostro 1550 Wi-Fi upgrade to 5GHz