Old iMac OS upgrade?

hello all. new posting forum, , question has been asked, couldn't find specific answer.

purchased 24" imac in december 2007: has become slow recently. have emptied half hard drive, cleaned out random applications , other clutter, cleared out login startup stuff, etc. still pokey. tedious spinning beachballs open app or web page. running 10.6.8 snow leopard. hw/sw footprint pasted below.

simple question: should upgrade os, , if so, yosemite? can old imac handle latest os x? improve performance?

if not, should upgrade recent version mountain lion or mavericks? (and how them? apple store makes latest 1 available.)

also, lots of useful apps nowadays won't run on 10.6.8.

appreciated, of course.
hotspur.

hardware overview:

model name: imac
model identifier: imac7,1
processor name: intel core 2 duo
processor speed: 2.4 ghz
number of processors: 1
total number of cores: 2
l2 cache: 4 mb
memory: 4 gb
bus speed: 800 mhz
boot rom version: im71.007a.b03
smc version (system): 1.21f4

system software overview:
system version: mac os x 10.6.8 (10k549)
kernel version: darwin 10.8.0

 

could failing hard drive. upgrading snow leopard won't speed on older hardware.

 

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