Check out the specs of the RouterBoard RB153.

It's actually a very neat little piece of hardware. Small, efficient, low-power. Only drawback is that the CPU is not able to power the full bandwidth the interfaces provide - thus if you really need 54MBit bandwidth - you should go over to the RB53x Series - which have the same CPU at 400MHz, able to handle the bandwidth.

• CPU: MIPS32 4Kc, 175MHz embedded
• Memory: 32MB SDRAM
• Storage: 64MB onboard NAND memory chip
• 5 x 10/100 Mbit/s fast ethernet ports with auto MDI/X
• 3 x MiniPCI Type IIIA/IIIB slots
• 1x 2.4/5GHz 802.11a+b+g High Power Wireless Mini-PCI Card (R52H)
• Onboard speaker
• Serial: One DB9 RS232C asynchronous serial port
• LEDs: Power, 3 LED pairs for MiniPCI slots, 1 user LED
• Power options: 11..60V power jack or IEEE802.3af power over ethernet (12V / 48V DC not for use with power over datalines)
• Power out: One 3V DC power output header, max. power output 500mA
• Power consumption: 3-4W without any addon cards. Max. 13W
• Dimensions: 160mm x 160mm (6.3in x 6.3in)
• Weight: 183g (6.5oz)
• Operating temperature: -20°C to +70°C (-4°F to 158°F)
• Humidity: 70% relative humidity (non-condensing)
• OS Support: RouterOS, GNU/Linux

You can have a view on the internals of mine.