harsha
if you are going for lower speeds,
better make 5-6 slots wtih IIC bus(SCL and SDA)and power supply lines(Vcc and GND) common for all.
since adc, dac, rtc, eeprom, lcd, and even I/O expander(2 to 16 lines expandable) is available in market, i feel its better to go for iic.
it will save your io's also.

i have designed such a kind of board with ISP options for 89c51rd2 and working well.
try
saji