First of all if gaming is the priority then 2500K will outperform the i7 950. SB at best offers x8/x8 where as X58 would offer x16/x16 but you would struggle to tell the difference in real life performance, benchmarking may give you a few percent increase.
Again the difference between dual channel and tripple channel RAM would be negligible in real life performance.
As in Post #9
Intel Core i5-2500K 3.30GHz (Sandybridge) Socket LGA1155 Processor £194.99
(£162.49) £194.99
(£162.49)
Foxconn P67A-S Intel P67 (Socket 1155) DDR3 Motherboard - (Sandybridge) £99.98
(£83.32) £99.98
(£83.32)
GeIL 4GB (2x2GB) DDR3 PC3-12800C9 1600MHz VALUE PLUS Dual Channel (GVP34GB1600C9DC) £33.98
(£28.32) £33.98
(£28.32)
Sub Total : £274.13
Shipping cost assumes delivery to UK Mainland with:
DPD Next Day Parcel
(This can be changed during checkout) Shipping : £9.50
VAT is being charged at 20.00% VAT : £56.73
Total : £340.36
The above is what I'd say is best bang for buck for gaming right now. Adding 4GB RAM later would improve your photoshop experience.
As for which motherboard, how likely are you to go for a dual card setup in the future? and also what card have you got at the moment/may get?
The Foxconn P67A-S Intel P67 is certianly the best bang for buck P67 board at the moment.
If photoshop is also a main priority then you'd benefit from getting 8GB RAM either now or adding 4GB later.