Firstly dont get another 4 GB RAM it wont boost your performance. That Quad is about as fast as you will get on that architecture. The PSU should handle the GPU. The only thing worth upgrading to is the 6850. If you want a major boost you have to go i5-i7.
Every test I have ever seen failed to show any real difference in performance by jumping to 8 GB RAM. Maybe if you had a dozen applications going at once then you might see something but apart from that nothing. Any gain that you might make under very limited circumstances would be as nothing compared to the gain you would get by upgrading to the i series. The GPU upgrade may give you a better gaming experience but that is all. The Core Series is old and yours has reached its peak. An SSD or faster HDD may help it but RAM...save your cash.
I agree with Malmental though, upgrade the graphics card (and the RAM if you feel it's worth it) and don't worry about upgrading the rest of the system, it won't be worth the money you put into it. The money could better be saved and used for a new system in a year or so.
with GTX460 being close to $200 I think its a better option than Radeon 6850...
Right now, Tom's Hardware has them listed as a virtual tie (GTX 460 1 GB vs. 6850). I would just get whichever you can find the best deal on. If you find a 6850 for less than a GTX 460, go for it. If you find all of the 6850s are more expensive, go for a 460.
I checked out the RAM article and it was very interesting. It seems to me that it is still not worth it to have more than 4 GB RAM. I had a Core 2 Quad with 8 GB RAM and an i7 with 4 GB RAM. Both had Velicoraptors for HDD's. When it came to rendering the i7 won hands down and it only had half the RAM. I never really noticed any major advantage to the 8 GB configuration. Indeed it made my boot times longer as the system had more RAM to deal with.