If you want to extend your network to a single direction, the WLE-MYG is a directional antenna that's cited to increase wireless range up to two times. We used it with several 802.11g routers and found the range improved drastically - in the intended direction - over distance. It would be ideal for extending your network to the garden.
For boosting signal in a more general area, you need a omni-directional antenna. The Buffalo WLE-HG-NDR is a high gain 6.5dBi antenna, which you can place on a desk or mount onto the wall. Like the WLE-MYG, it costs around £40.
Both of these antenna can be used with 802.11b and g networks. Unsurprisingly, Buffalo's antennae are designed for use with its kit, but the packs include a MC to RP-TNC connector that enables you to use them with a variety of devices. Dan Grabham