I'm not sure if the upgrade is really worth it. If you want to get a performance increase maybe consider adding 2-4gb more ram, reinstalling windows on an ssd will give great improvements for effeciency and speed. Overclocking your processor current processor would be good (I recommend adding aftermarket cooling), you will notice nice gains by doing all of this.
In my opinion don't half arse things and if your gonna add something to a pc, it should only be a new gpu, ram or storage capacity. If you feel the need to upgrade your cpu it's probably time for a new pc. And with your current cpu I believe you still have a decent pc.
[h=1]Corsair Force Series 3 SSD. $169.99
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[/h][h=1]G.SKILL NS 2GB 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) X2. $25.98[/h]Newegg.com - G.SKILL NS 2GB 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Desktop Memory Model F3-10666CL9S-2GBNS
[h=1]CORSAIR CWCH60 Hydro Series H60 High Performance Liquid CPU Cooler. $74.99[/h]Newegg.com - CORSAIR CWCH60 Hydro Series H60 High Performance Liquid CPU Cooler
The good thing about that liquid cooler is that is requires no maintenance, ever... It shall help you overclock your current cpu to a higher frequency.
And the advantage of this is, all these components can be transferred to a new pc easier if you decide to build a newer one in the near future. You may need to purchase windows if you don't have your old copy lying around that you can reuse.
Good luck though!