What is the best networking products for my internet connection?

I can't decide what is the best products to improve a better connection and speed for my internet. Also my family members use wifi for their laptop and ipads. I dont have any experience of networking and I cant decide what I need to improve on either buy a new modem, wireless usb adapter or motherboard with wifi. I dont want to buy one of each and just want to know what is the best solution, my budget is $350 slightly more or less is fine with me.

D-Link DSL-2870B N750 Wireless-N Dual Band Modem
Specs:
• 802.11b/g/n standards
• Wireless speed: up to 300Mbps
(2.4Ghz) & 450Mbps (5Ghz)1

I have a ASUS USB-N10 NANO Wireless N150 USB Nano Adapter
Specs:
802.11 b/g/n
Data Rate: 150Mbps

Also I have a decade old motherboard which I want to replace. Im not sure if its necessary to have a motherboard with wifi or just a wireless usb adapter.
Added (1). Also I have a ADSL2 connection

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