First off Tiger is OSX 10.4.X
Next are Mt Lion Requirements:
So here is the big one, if you have Applications that are PowerPC based (MS Office 2004 for example) they will not work in the MT Lion OSX. 2GB RAM is a bare minimum (which you have) but you should probably max out your iMac with as much memory as it can handle. Crucial or OWC have complete lists of which machines can use what memory and how much. The 8 GB of Hard Disk space is also only the space required for the OS is also only a minimum standard as you should keep about 10-15% of the Hard Drive free for Applications to read/write into to operate properly.
Memory- RAM (Random Access Memory)
Memory- Hard Drive space Total, Used and Free
Speed - Processor speed The Speed at which the CPU (Central Processing Unit) completes its operations
Bus Speed is the speed at which the computer transfers signals throughout the system