the bit rating is inconsequential. Don't pay any attention to it in terms of COMPATIBILITY. The memory bus width, expressed in bits, is an indicator of overall performance. Right now, the norm for high end GPUs is 256 bit or more with GDDR5 memory. 128 bit with GDDR5 is still OK, and 128 bit with GDDR3 is going to be what I'd consider a "mainstream" card.
Power supply would also need to be upgraded to support the 5570, yes. Fortunately a 420 or 450 watt would be enough for that card, so look for the Green power models from Antec in that watt range, they tend to be inexpensive, but high quality.