realistically, you won't see any difference in "normal " day to day usage
Whilst the headline download speed might say 300mb, in the real world, that would be between a server sitting beside your computer and your computer.
You might see an actual download speed on a large file of between 100-150mb but that would depend on the server having a large bandwith and no one else downloading from it at the same time

 

In other words, does a usb wireless ac dual band adapter work independent of all other network devices on one's PC to provide the best Internet speed or is the speed still throttled by one's PC internal NIC card limitations.

Depending on age of the laptop it should default to the new wifi adapter....if not you can turn off the internal one. On laptops there is a FN key combo or a wifi button to turn off. If there isn't one (should be) you can turn it off in the BIOS.

What is the make/model of the laptop in question?

 

If you have more than one connection (with a Default Gateway specified) assume that Windows will pick one at random for internet access. The easiest and surest way to get what you want is to disable the unneeded connections (as bassfisher6522 said).

 

If I want to use a more modern USB wireless adapter in a laptop that has an older built-in wireless device, I disable the built-in one in the Device Manager first before inserting and using the USB one.

Even with a more modern USB wireless adapter, you likely still won't get anywhere close to your advertised 250 Mbps download speed.

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Thanks all for the great explanations and advice! In summary I'll buy a new usb wireless adapter, disable my internal wireless card and we'll see how fast it goes As an FYI, a friend signed on with his new iPhone 6S and using the speedtest.net app he showed download speeds of 220mb. The iPhone WLAN card is Wi-Fi 802.11 a/b/g/n/ac, dual-band, hotspot so I'm hopeful

 

Also note that the Router MUST support the higher speeds.

How do you connect to your ISP?

 

Thanks DaveA. The router should be fine. It's supplied by the ISP as part of the new service plan. Specifically it's the ARRIS DOCSIS 3.0 Residential Gateway with 802.11n/ 4 GigaPort Router/ 2-Voice Lines for Comcast (TG862G-CT) http://goo.gl/LgqYUT It's connected to the cable wall outlet.
What's nice is it allows me to also connect my own Asus Router - ASUS (RT-AC68U) Wireless-AC1900 Dual-Band Gigabit Router - http://goo.gl/o3WVKM allowing me to setup two home network SSIDs ... well technically 6 is you take into account dual band and guest networks

However, I may purchase my own cable modem gateway router if everything else works well so I don't have to keep paying the 10/mo fee ;-) Especially as I don't need the voice lines feature.

 

My ISP provides me with pretty much the same model for my 200 Mbps service, but they charge me $4.00 a month rental.

If you do decide to buy your own, make sure your ISP supports using it.

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Thanks Flavallee, I'll definitely keep that in mind

 

You're welcome, and good luck.

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On a related note ... now that I'm trying to find the best money/value WiFi USB adapter for my needs, came across a nice comparison review:
"Tested: We push six 802.11ac Wi-Fi USB adapters to the limit" http://goo.gl/dbvZgF
Looking at the results it's interesting the Netgear usb 2.0 adapter outperformed all but the Asus usb 3.0 adapter with a price tag of $30 compared to $70. Looking on Amazon there's even a manufacture refurb for only $14! As my laptop has two 2.0 and one 3.0 usb ports, I'm wondering how much the usb 2.0 vs. 3.0 option will effect internet download speeds, taking into account the plans max speed advertised is 250mb? Perhaps I'll buy this netgear $50 wireless usb 2.0 adapter and see how it goes No doubt the devil's in the details
NETGEAR AC1200 WiFi USB 2.0 Adapter - AC Dual Band (A6200-100PAS)
It claims to handle 300/900 2.4g/5g.
http://goo.gl/BbIs13

I'm also looking into getting a new ethernet gigaport usb 3.0 adapter to handle those times I need the fastest possible speeds by connecting my laptop directly to the router via gigaport ethernet.
Here's one I'm looking at:
"Anker 3-Port USB 3.0 HUB with 10/100/1000 Gigabit Ethernet Converter (3 USB 3.0 Ports, A RJ45 Gigabit Ethernet Port, Support Windows XP, Vista, Win7/8 (32/64 bit), Mac OS 10.6 and above, Linux) Black"
http://goo.gl/ptxGSs