Maybe a used q9550, q9650, or a modded Xeon x5450 or higher would be worth it. If your motherboard can handle a 1333mhz FSB CPU then it would be worth your time and money to go for one of these.
There are a few caveats to the q6600.
The q6600 is a 1st Generation Core 2 CPU but your e5200 is a 2nd Generation Wolfdale/Core 2 CPU. Your Pentium CPU is like a dumbed down Core 2 Duo e7400. The e5200 still has a higher clock speed and 10% better IPC performance. You'd be trading single core performance for multithreaded performance.
In some cases the extra L2 cache in the q6600 would help shore things up and even beat the processors with lower amounts of cache but not all the time.
800mhz RAM was pretty normal and standard for those processors. Those old Core 2 processors really didnt make good use of memory speeds that were any faster than DDR2-800. Probably because the memory controller was on the Northbridge chip that's on the motherboard.
Yes, upgrading to a q9650 would help a lot if you also have 8gb of RAM and Windows on a SSD. This would work great for running Windows, virus scans, surfing the web, watching videos, and playing older games if you have a GPU.
NO! The q9650 is showing it's age and this shows because some newer games and more advanced programs would run like crap with it.