My advise, since I was in the same though process few days/weeks ago:
1. I went for the 860 due to the multithreading that is supposed to help on multitasking. No complains so far, but no idea how much better it is compared to the i750.
2. I am using 4Gb of RAM, but do find PS to slow down a bit when I work on 2 RAW files in Photoshop(I allocated 70% of RAM and a dedicated scratch disc).
3. I am using 4Gb RAM with 5850 with an old 520W PSU.... So far, it's doing ok.
If price is not an issue , the i7 will definitely out perform i5 , but the performance differences will be more visible if you do processing hungry multitasking .
For LR applications & CS4 and office application , multitasking between them would probably be not significantly visible.
I7 will steer away as the clear winner when it comes to 3D gaming.
However , not sure are you talking abt the onprocessor video capabilities . It will be prefered to have an GFX card to play games. IF using a third party GFX , the video performance will largely be dependent on the GFX processor you have. Multithreading will probably not be an issue for gamers , unless you are those that like to switch between applications while in games.
In terms of overall cost - performance , i5 has better value from most reviews if you do a search on the internet. No real need to jump on to i7 unless we really need the extra processing power. The 2 processors you mentioned works on the same LGA1156 motherboards , and both runs on DDR3 only ,hence rams are pricier compared to DDR2 for non ICore processors.
I am currently using a I3 530 with H55 mobo with 2Gb Ram. Applications used often are LR , CS4 , IE and Office. Video card wise I m using an ancient GF7600 GT 256mb. So far has been doing me well for my PP processing. No significant lags experienced editing raw as well.
Will do an upgrade of RAM and GFX later when Diablo3 is released lol =)