Er... 1-sided RAM means that you can only find a number of chips (usually 4 or 8) on a side of the DIMM module but none on the other side. 2-sided RAM means that there are chips on both sides.
For DIMM module, 1-sided RAM means that it'll occupy 1 memory bank on your motherboard's RAM slot, while 2-sided RAM occupy 2 memory banks. However, 1 DIMM module (no matter 1 or 2-sided RAM) uses up 1 RAM slot on your mobo.
The differences between "1-sided / 2-sided" rams are as follow:
1) 1-sided RAM usually has denser capacity compare to 2-sided RAM (eg. 512MB RAM = 64Mx8 on 1-sided RAM = 32Mx16 on 2-sided RAM)
2) 1-sided RAM usually has higher latency compare to 2-sided RAM, due to larger capacity. (Those premium RAM usually comes in 2-sided RAM)
3) 1-sided RAM is harder to manufacture, due to larger capacity
4) 1-sided RAM use up less memory banks on your mobo. Some mobos cannot support 8 memory banks, which means that you can't use 4 2-sided RAM at once.
I guess that's about 1-sided/2-sided RAMs
RAM Handbook, updating ... zlol 23/12/07