zero padding

i m designing an OFDM transceiver in xilinix system generator. i took data and modulated the data using QPSK. then before giving my data to IFFT i need to do its zero padding. my incoming data is at the rate of 800 samples per sec. i want to take 1st 400 samples and store them. i tried to use RAM of depth 400(i dont know i m right or wrong), then next 400 samples in another RAM. and now i want to add 224 zeros in between these two data samples of 400. My data rate has been increases from 800 samples/sec to 1024. How can i store this higher sample rate. and how can i implement my problem in system generator. can i use FIFO any where.