Whether you use streaming or memory-mapped doesn't necessarily matter.
If you are using the NIOS with the InterNiche Stack, then once the packet is received, the data is already in memory. You can start just reading the data from memory and sending it to your processing block. You could do this either directly in the NIOS or using a DMA controller.
If you are not going to use the NIOS, you may not need to buffer the frame in external memory before processing it. You may be able to just send the data to your processing block as it comes in. This depends on your system design.
Jake