TX power can be set by changing the CRDA to a country that has a higher power allowance. Like Bolivia:
sudo iw reg set BO
- It's illegal to have unlicensed over-powered equipment in most countries.
- It doesn't help that much. It'll only help if you're pushing the range to the limits.
- Some hardware isn't designed for this level of power and so isn't sufficiently cooled. This could break your equipment.
- It's also driver and hardware dependent. It won't work for everything.
For everything else connection quality is paramount. Assuming the driver is fairly solid, you want to be looking at physical things:
Can you use a directional antenna if you're talking about two static devices? What about DIY tin-can reflectors or cantennas? They work but they're only good for static point-to-point.
Can you fit better aerials to each end? Big juicy aerials.
Don't use wireless extension cables. You lose ~1dB per meter .
Position your router in the centre of where you want to use it. If that's not enough, there are fairly cheap wireless extenders now.
As you've just said in your comment, avoid other 2.4GHz devices. Phones, baby monitors, remote controlled planes... Essentially anything wireless.
Scan your house for other people's routers' signal. Find out what channels are busiest, Use the least popular one (allowing at least two channels either side).
Do you need wireless? If your key interests are static desktops, consider things like ethernet and powerline. However much hassle installing it is, you just can't beat wires for connection stability and throughput.
If the driver isn't any good (eg you get much slower than it says it should support), consider upgrading to a better chipset.
In a lot of laptops the wifi card is just a mini-pci board that can come flying out and find itself in the bin. You get get nice, supported, tri-channel (450mbps) Intel cards for a surprisingly low price.
You could fight to the death with the driver but the manufacturers clearly consider it junk if they're not going to support it. You should do the same.