Increasing wireless bandwidth

[x El Scorpio x]

I use a ZyXEL Prestige 660HW-D1 at home and it has only one antenna with the wireless bandwidth rated at 54 Mbit/s and the problem is that I will be using this connection for 2 PCs an Xbox 360 and a Laptop (not usually all at the same time but often 2 or 3 devices are running at once). The router also has 4 100 Mbit ethernet ports, which I can sadly not connect to any of the devices as it would be somewhat complicated to set up the cables.

I have looked at several options of increasing the performance and think that perhaps the best option would to be to connect a wireless bridge (access point) to the router with an ethernet cable and set up some devices to use the router's wireless connection and the others would then connect to the access point. Another option might be to purchase a better antenna for the router but I am not sure if the bandwidth is restricted by the standard antenna or the router itself so I am not sure about this. Alternatively I think it could be possible to replace the router with a better one with several antennas (for example the ZyXEL NBG-415 with 300 Mbit/s WLAN bandwidth) but that may be a bit problematic as the router was configured by the company that my dad works for and we might not have all the necessary details to set up the ADSL connection...

So what do you think would be the best idea to increase the bandwidth without having to spend too much money?

Thanks in advance for any suggestions you may have!