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I have a 2 floor house with a basement. My router is located in the basement and my bedroom PC is located pretty much as far away as possible at the opposite corner of the second floor. I use an internal PCI-Express 2.4/5 Ghz WiFi receiver card with three antennas, so I ordered an extra package of these to replace all three antennas and give me a spare.
The result is that I went from 0-1 bars of 5 GHz signal strength with 10-25 MB/s transfer speed from other PCs on my home network to 2 consistent bars of signal strength and 30-40 MB/s transfer speed from other PCs on my home network. Internet speed tests on my 120 Mbit connection used to get up to around 115 Mbit/s or so and then gradually drop off to settle around 80-90 Mbit/s. They now cap out at 120 Mbit/s and stay there. I didn't check my before/after packet stats but it's probably safe to say I'm dropping fewer packets now as well.
Unfortunately my router's antennas are all permanently connected or else I'd try replacing them too. I think if I'd been able to replace those as well I might've been able to get even better results, maybe 45+ MB/s or so.
My router is an Asus RT-AC66U_B1 and my PCI-E WiFi card is a Rosewill RNX-AC1900PCE I got from NewEgg, which is actually a rebranded (and 1/2 price) TP-LINK Archer T9E. I highly recommend the Rosewill NIC or its TP-Link equivalent if you get get it at a good price. I've only had it a week or so but it's been absolutely reliable with the TP-Link Archer T9E driver installed as reviewers on NewEgg recommended. Maybe the Rosewill driver is fine too, I never even tried it.