imac 2008: to upgrade or not?

hi as the proud owner of a 2008 imac i ask how worthwhile to squeeze value remaining of this investment.

it freezes every week or so requiring reboot, but its overall performance is adequate for my needs.

Might increasing ram from 4 to 6GB and upgrading a conventional 7200rpm internal hdd to a hybrid model significantly improve

reliability or is it far wiser to use the money instead toward the inevitable purchase of a newer imac?

-david