unlocked multiplier means that the on chip multiplier is fully adjustable. typically when you have a chip rated for a certain FSB (most current FSB's or reference clocks are 200). Manufacturers use a multiplier to set what the actual speed of a CPU will be. Since FSB's are stock at 200, a CPU with a 10x multiplier clocks at 2.0Ghz without overclocking. The 10.5x multiplier on my HTPC clocks it to 2.1GHz. these multipliers cannot go higher on stock chips, which is why we adjust bus speeds (reference clocks) to get more speed out of a CPU. A chip with an unlocked multiplier can go up, say it's rated at 10x, you can take it up to 11, 12, or however high you can push it. Overclockers like unlocked multipliers because it takes less bus speed to reach a higher clock speed.
Cache size is memory that is built into cpu's that store the data cpu's are calculating at a very fast speed. it's kind of like a storage space for intermediate calculations that don't require an output. Cache is a lot faster for the CPU to access than system memory, and having more cache can be seen as a performance improvement.