The best answer here is to put into this what it is worth - which is basically a low-end gaming system by today's standards. Dropping one of the hexacore CPUs into this will likely represent an investment simply not worth the returns, since those CPUs are highly sought after now that they are out of production and command a high price on a resale circuit - so much so that newer, cheaper chips from AMD and Intel beat them out.
However, given that the above is not actually the ANSWER to your question, the ANSWER lies on the CPU support page for your mobo's revision level: the most powerful processor available to you is the Phenom II X6 1090T. The AMD Phenom II X6 1100T May also work, but it is not officially supported by the BIOS, so I can't say definitively that it will work. It will probably work. Be careful, because there are Phenoms made which exceed the TDP rating for your board and could easily damage it.
What GPU will be bottlenecked/not bottlenecked by this CPU? All of them; that's not how bottlenecking works. Different programs place different strains on different parts of a computer, so even a high end CPU can be the bottleneck of a low-end GPU if it is trying to do something like downscale an h.265 video transcode, which is highly CPU-intensive, or play a game like Civilization 5, which is notorious for its high CPU usage in the late game but places relatively low demands on GPUs.
Because the CPUs available to this board are somewhat weak compared to modern offerings in a similar price bracket, it makes sense to get an Nvidia GeForce GPU instead of an AMD Radeon GPU, because historically Nvidia driver overhead (the load placed on the CPU when the GPU is running) has been higher for AMD products. This is not a major consideration, however, so price is a major determinant.
The onboard GPU is of no consequence for your purposes unless you want to attach a second screen to it; then it would make a little more sense to go with Nvidia for your primary GPU just so you could have a driver environment all from one supplier, which should be a little more stable (not necessarily though, since you will need to run two separate drivers and that old motherboard GPU may not have Windows 10 compliant drivers).
With a 1090T CPU and a decent overclock to something like 4ghz on all six cores, given a Titan X GPU (not a recommendation, just a removal of most GPU bottlenecks for hypothetical purposes); this system should be able to handle most modern games at maximum settings; a notable exception would be any title which is heavily single-threaded and demanding on the CPU (most bad console ports, for example, of which Assassin's Creed: Black Flag and Unity are shining examples). In general, the CPU is less a limiting factor in gaming PCs than the GPU. Tests have been done showing the lowly Athlon 5350 performing admirably when paired with a midrange modern GPU. Where you are comfortable in that gradient of performance is largely up to personal taste.
