Is an Arduino capable of managing a 3D printer and additional modules?
The Atmega Arduinos lack the power to do any serious computation. Another shortcoming is the low number of interrupts you can use. (8bit @ 16MHz, 2 external IRQ)

The new Arduino Due has a much stronger CPU, the same form factor and price as a Mega, and can handle more IRQs. (32bit @ 84 MHz, as many IRQs as pins)

One problem is the change in I/O Voltage, which is now 3.3 V. RAMPS should be working with minor tweaks, see here 

Then again, you may want to do the heavy lifting on your computer, or any one of a number of ARM dev boards, and just send the g-codes to RAMPS. Look at Raspberry Pi or maybe STM32F4discovery, or netduino for that matter.