If you only care about that PS3 coming into a better signal quality from the AP, use a highly directional (so high-gain) antenna pointing to your PS3.
I do not know the law on your country, but here in Spain you have to lower the signal strength at the emitter to use a high-gain antenna. BUT if you do not live in a country with such regulation you can use that method to increase transmitting and receiving power to and from your PS3.
Please, notice that using the antenna would prevent almost anyone else from connecting into the AP since all power is transmitted towards the PS3.
This antenna should to the job, but, please have a look at your country's regulation to find out if this technique I am describing is legal or not.
Or you could use three of any of these (because of 11n MIMO technology) pointing exactly on the same direction and separated from each other exactly the same distance as antenna connectors are on the DIR655 device.
Here is a high-quality antenna manufacturer:
There are higher gain antennas out there you could try to find, but I would suggest you use a single antenna with three connectors to plug into your DIR655 instead of the three pointing at the same direction.
Also, the higher the gain, the harder to focus onto the right direction.
Please, again, beware local regulations and do this at your own risk. If any regulation is broken or you cannot manage to do it properly, I take no responsibility at all.