But before embarking on any purchase I suggest to make some checks. Because in almost all cases, the problems begin at the router configuration. In search of the owner's manual of your router, then!
Then it is useful to download two programs; (Vista) Stumbler and Wifi Analyzer, a free app for smartphones.
VistaStumbler lets see wifi networks around you, especially the channels used by them. A priori channels 1, 6 or 11 are preferable for the internet. Depending on the result announced by VistaStumbler, one can choose on its router the least usual channel - or verify that the router configured to optimize itself choosing the best channel is not found consistently on the busiest channel.
For that matter, check the configuration by 802.11g, b, or even b. In my case, having replaced the multiple choice with only 802.11g (not 'n' wifi because my printer does not support it) has probably made a very slight gain speed, but has mostly solved a very big network instability.
Last step; the router's location and / or direction of its antennas. The game consists of measuring the power of the wifi in every room with Wifi Analyzer, change the positioning of the router to re-measure ... and finally to opt for buying a TL-ANT2408CL antenna if (and only if) the wifi signal is present in the room, but too weakly.