I replaced the stock antennae on my 2800G with two of these:
http://www.dsl-warehouse.co.uk/product.asp?pr=VIGOR5DBI
These are Draytek hi-gain antennae that are suitable for all the 2800-Series Draytek routers (to the best of my knowledge).
My router is around 6 metres away from my PC and there are two walls in the way. I get a 108Mb/S 'Excellent' signal strength connection to the 2800G using these two antennae.
With the stock 2800G antennae I get, at best, a 'good' signal that usually drops to 54Mb/S.
Highly recommended. By the way, the WiFi card on my PC also has an external desktop antenna to improve the signal.