There are a number of steps you can take to improve WiFi performance without changing routers, the HG635 is generally a solid performer. As an avid follower of the community I am sure you must have seen many posts on this subject recommending such things as using a wireless scanner to check for congestion, have you tried that?
Yes I have and have found that the signal drops rapidly anywhere upstairs - and "beamforming" does not appear to help.......in fact with beamforming off - the signal is marginally better !
I have tried "pointing" the router towards the upstairs PC and that makes no difference at all
It does seem a waste of a fast connection (and fast i7 PC) not to make use of its full potential