32bit OR 64bit ??? & what must be the burning speed ???

An i5 is a 64-bit processor, so you can use either the 32-bit version or the 64-bit version of Ubuntu. I recommend you use the 64-bit version, as that will better take advantage of your hardware. As for the burning speed, when burning something like software (especially an operating system) where a single bit error could prevent it from installing or running correctly, it's a good idea to select the slowest speed available. However, so long as your drive and (this is the important part) your CD-R media support it, you could use any speed. (If you burn media at a speed higher than it's rated for, then the resulting burn has a high likelihood of being corrupted.)