Could be many things.
Wifi interference, bottlenecks inside the devices (hard-disk, CPU, or whatever), and probably other things.
Start by surveying the WIFI frequencies and make sure you have the best one. Try several ones. Also start with a test setup really close to the wifi access point. This should remove the factor of interference.
There are also wifi settings accessible on the wifi access point, for example modulation types, whatever repeat whatever and many other settings that you can change (my memory is on vacation). These settings are a tradeoff between speed and reliability. If you have a clean setup (no interference), you should be able to maximize speed using these settings.
One more thing: 600KB/s transfer is more than enough for streaming youtube 720p, and usually 1080p without a problem, so I would say it's already good.
If you really want the edge in HD, why not use ethernet?
PS:
NuclearVision wrote
Could this be a problem, my router? I tried using another router, the thomson sold by ogero. Same results: i believe this one is only 54mbps.
If your adapter does not support whatever 300mbps mode your router supposedly uses (we're not even sure that mode is enabled) then it won't work at that speed - most probably switch down to some slower compatible mode. It could also be the encoding or software that you're using for video that is inefficient. You really need to look into the details and see what is happening to find a solution - you're best placed for that.