Can I use 1600mhz RAM instead of 1133mhz RAM for my laptop?

Yes, you should be able to use the 1600 mHz RAM if you cannot get the RAM of specified frequency. RAM does not run at the rated frequency. The speed depends on the mother board.

The RAM rated speed is the maximum that can be achieved with the particular RAM. A 1600 mHz RAM will run at a speed of 1600 mHz or any speed lesser than that. The actual speed at which your RAM runs will be determined by the combination of your motherboard and processor.

Some good options could be -

1. Transcend DDR2 2 GB PC DRAM

The RAM is available in 2 GB capacity. It is priced at Rs. 1160. You can get it on Flipkart.

2. Kingston 4gb Hyperx Fury 1866mhz

It is priced at Rs. 1842. You may buy it from SnapDeal. The RAM is DDR3 architecture.

3. Adata Premier Ddr3 1600 2gb Ram

You can buy this DDR3 RAM in 2GB capacity, It has been priced at Rs. 949. You can buy it on SnapDeal.

RAM speed is limited to the frequency supported by the motherboard. So even if you get the RAM with higher frequency, motherboard only makes use of the calibrated set of frequency. Also the 32-bit or 64 bit doesn't cause any issue for the DDR3 RAM. Both these bit architecture is compatible with DDR3.

Lenovo also has the custom upgrade section that you can access online or in service center. This way you can easily upgrade to the RAM supported by your motherboard.

I found these three RAM suitable for your case.

1. Crucial 4GB CT51264BF160B 204-pin SODIMM DDR3 PC3-12800 memory module. You can get this one here:http://www.amazon.in/Crucial-CT51264BF160B-204-pin-SODIMM-PC3-12800/dp/B005LDLV6S/ref=pd_cp_147_4?ie=UTF8&psc=1&refRID=0W03VSRS0VT69K100A10

2. Kingston 4GB Module - DDR3L 1600MHz CL11 , 1.35V Low Voltage Laptop Ram (KVR16LS11/4): You can get it here:http://www.amazon.in/Kingston-4GB-Module-1600MHz-KVR16LS11/dp/B00CQ35GYE/ref=pd_cp_147_2?ie=UTF8&psc=1&refRID=0239HPGHW2QC838X0QVX

3. Transcend 4GB DDR3L 1600 LAPTOP RAM:You can get the module here: http://www.amazon.in/Transcend-4GB-DDR3L-1600-LAPTOP/dp/B00B5LOOJG/ref=pd_cp_147_1?ie=UTF8&psc=1&refRID=0239HPGHW2QC838X0QVX

Yes, you will be easily able to use 1600mhZ RAM with your laptop because it is completely dependent on motherboards. So it will be same as before.