I've said this for years now.
Know one can tell you you will get X an increase if you buy a well know brand. But I will say this don't beleive you will get a better performance if you buy a router sponsored by Sony, Micorosoft etc..as the router company pays for thier support.
I will say this: if you swap your cheap £20 ebay "chinese" router for a well know brand, Belkin, Netgear, linksys etc. You will see a more consistence speed, less dropped connections, less errors and better a wireless signal (with less droppped connections).
P.S. the routers given by ISPs arn't usualy the greatest either - maybe a good name but has the ISPs firmware.