Any help appreciated!

there is not a lot you can do. even upgrading the cpu wouldnt give you much in the way of better performance.
but there are things you can do to smooth things out.

use hdd tune to check yours and i guarantee you will be lucky if you get 100MBps max and 80 average. this will likely be the biggest performance choke point on your system.

raid 0 will double your data through put allowing faster uptake of things like textures over all faster read and write speeds.
an ssd is an option but they are expencive for what you get and because you have sata 2 the difference would be nominal over a good raid setup.

i run 2 spinpoint f3s in raid which gives me 2tb of really fast storage for £100/$150 where as a stock 80 gig hdd will cost just as much but only give 1x20th of the overall capacity.
you could of course use a pci-e sata 3 controler to give up to 600MBps transfer speeds but again its expensive for what you get.
you already have a 1.5 tb drive but if its a 5400rpm then it will cripple any performance gains you hoped to get.
instead of using that as your primary get 2 spinpoints f3's (the fastes sata 2 drives available) of 500-1tb each and set the 1.5 as storage only. it will increase gaming performance more than a cpu upgrade from what you already have.

some tips...
if you have more than 2 partition on your current drive then get ridd of them each 1 slows data transfer by 10% for each. but you will want a minimumof 2. 1 smaller as the primary 250-360 would do. the rest as storage.
doing it this way means you dont slow your pimary partition by having files littering it that you dont use but will slow down drive performance.
on my setup i have 2 partitions 1 primary 500tb which runs at up to 250MBps and the rest is fast storage. thats 1.3 gigs of space that runs at roughly 180MBps...
if you have a 3rd party cooler then learn how to over-clock your cpu... try to stay within stock voltages and you wont reduce the life expectancy of the cpu...
clean your system with a good cleaner like ccleaner to clean out your startup of unneeded entries like itunes if you have em. and optimize with something like windows 7 manager (use its optimization wizard to shut down any un needed services and processes.)