I upgraded my late 2008 MBP to 4 gig of ram when I first bought it. Is it safe to upgrade it to 8 gig? (Crucial said I can). Will it make a different in speed. Seems kind of sluggish with just chrome, twitter, and iTunes opened. ps- can't do SSD because I just got a new HDD last year after my hard drive died. Thank you for any advice.