RAM, Graphics card and Power supply Unit(PSU)

hello there,
I have been thinking about buying a customised desktop PC for home use. But i can never decide on the PSU and the graphics card and i don’t want to go over £750. At the moment i got a Dell laptop with 2gb ram and 1.8 dou 2 core inspiron 1525, which seems a little slow now.
What system would be best for my needs? I use my PC mostly for web surfing and downloading stuff and everyday application like photo and video editing on occasional basis, don’t play games at all but can never say never still got some pc games that i love like Hitman2 etc, got ps3 but the games cost allot and i cant be asked playing on that... 
Is it better to have a bigger PSU and and would u say a regular 450wat 80+ PSU would be ok for my specs below?
Intel I7 , or possibly I5 quad core
Nivada Geo graphics card not sure which is the best value for money and performance yet so could do with some tips on that ( been looking at GT640 and 650 but they seem to be same to me apart from one 2gb and the other 1gb)
RAM, how big ram should i get 16gb? I mean would it influence my performance if i was to add more ?
USB 3.0 function, Wifi 300mbps be ok?
Can Blueray rw can also write dual layer regular dvd dual layer?
And what are some really good motherboards?
SSD, should i add one ? i heard they got short life spam and costly and what if i was to just specificly use ssd for OS only so that way it wont run out of memory so soon and not too much of wear and tear
And finally i would like to add that my aim for building this machine would be speed and performance...