The 15" 2008 Late can handle 8GB of RAM (Actual) 4GB (Apple)
The 17" 2008 Late can handle 6GB of RAM (Actual) 4GB (Apple)
Switching internal boot drive
Use a external Firewire Disk Utility HFS+ Journaled formatted drive (equal or larger than your new drive) and Carbon Copy Clone your present boot drive to the external.Repair permissions on both. Hold option and boot from the clone.
Switch your hard drives, option boot from the clone and follow thesteps above again to reverse clone onto the new drive.
Piece of fringing cake, and your drive gets optimized in theprocess.
Use the free OnyX to clean your system, optimize and maintenance,run all the cleaning and maintenance and reboot.
iFixit has instructions, OtherWorld computing has internal drives and tools.
Crucial.com has your exact RAM specs.
Have at it. Use the external drive as a backup clone.
(no comp for site/product mention)