@sapphiron:
Most routers/ADSL modems are limited to 100mW output.
Secondly, the AP that you gave does not include an ADSL modem.
Lastly, simply pushing up the transmit power to 1000mW is just silly. You don't necessarily get a better signal that way. All you're doing is overriding the other signals in your area and generating a lot more noise.
Using high gain antennas is always better than just boosting the transmit power, because the antenna gain has a positive effect on both the AP and its client(s). The transmit power has only an effect on the client(s).
@Dratsab:
For the best results upstairs, you should place a router/WiFi AP upstairs and use a LAN cable or EoP (Ethernet over Power) device to connect it to the router/modem downstairs.
Although high gain antenna's are beneficial to both the AP and its client(s), it also limits the area where reception is possible.
The high gain antennas have a narrow band in which it sends/receives, so you'll have to point it perpendicular to the direction where you want reception.
eg. if you want reception on the same floor, then you point the antenna vertical. If you want reception on another floor, then you point it vertical with an angle.
If you already have a modem with a WiFi AP downstairs, then you can always go for a WiFi range extender aka WiFi repeater, if you don't want to use LAN cables to connect the 2 floors.
Here's a good guide on optimizing your range/throughput:
http://www.lmi.net/wp-content/uploads/Optimizing_802.11n.pdf
* it is aimed at 802.11n devices, but it is still applicable to other WiFi technologies.
Lastly, if you don't already have an ADSL modem, then I would also vouch for that TP-Link W8960N like ichigo suggested.
Another option would be the D-Link DSL-2750U / DSL-2760U.
If you already have a good ADSL modem, then you can consider a router/WiFi AP that does not include an ADSL modem - like the one that Sapphiron suggested. Scoop Distribution does not ship, and their prices does not include markup or VAT.
Uniterm Direct's prices excludes VAT, but it includes their markup and they do ship and sell directly to consumers.