It depends on the type of usage. If gaming is your primary usage, then upgrading your GPU first would be the way to go.
That being said, and keeping in mind the specs you’ve shared with us, you may want to wait a little longer and upgrade your GPU, CPU and motherboard. With the holidays around the corner prices for said components will drop and/or bundle packages will be offered by various retailer/eTailer (i.e. Newegg.com or TigerDirect.com).
If you’re looking to upgrade your CPU now, you’ll need to look for AM3 compatible CPUs (as ksham has mentioned). You’ll want to look for Athlon II series processors, for example the Athlon II X2 270 3.4GHz: http://www.newegg.com/Product/Product.aspx?Item=N82E16819103953
(it’s listed for $59)
By the way, here the complete list of all AM3 compatible processors: http://products.amd.com/en-ca/desktopcpuresult.aspx?f1=&f2=&f3=&f4=&f5=AM3&f6=&f7=&f8=&f9=&f10=&f11=&f12=
About the GPU, you could jump to the 7000 GPUs, but to get the most performance out of it you may want to upgrade the GPU as well. Note, sticking with the 6000 series (for example a 6950 or 6990) may not give you a substantial boost in performance, here is a link to help you learn more about this series: http://www.amd.com/US/PRODUCTS/DESKTOP/GRAPHICS/AMD-RADEON-HD-6000/HD-6570/Pages/amd-radeon-hd-6570-overview.aspx
One final note, if you do choose to wait a little longer, consider going with an FX-6300 processor, or FX-8350, and a 7950 GPU. These are great for gaming and should help you keep the cost low.
Hope the above helps you…