Are you hitting a hard cap, or are you just getting roughly 650kb/s?

Roughly, but not over, so I guess 650 is the cap.. I've tried different methods of testing speed (speedtest sites and downloading files) but wireless just wont go over 650..

A hard cap points to cheap hardware. Either you need a better router, or a better wifi adapter - and possibly, both.

Download net stumbler, sniff out what channel other people are using and try a number about 3-4 above or below.

Failing that check different channels inside router.

Always test with encryption off, if you get a gain in speed then go back to WPA you can attribute a 10% loss to encryption.

USB devices work on an omni directional basis and I bet your router is too, so 2 walls is a lot to get though.
Consider a "pointy" aerial for best results. Or do something to make it direct as possible.
Or rig up something like a radar beind the router to channel in a more defined direction.

Zyxel modems are not suppose to be that great.
Its quite probably a bad relationship with the router/card.

But mostly walls/plus wireless/plus omni directional air waves is just not a great combination. Wired is still the best and safest for a desktop.

Also try a dif wireless card, or move router.

Thanks, think I'm gonna switch back to wired.. Have to choose between half my dl speed and no cable / full dl speed and cable running through the house.. not allowed to drill holes in the walls.. and have now wasted 30 euros on a crap adapter.. arghhhhh