If you're building yourself, you should get a xxxK processor as you can easily get 15-20% extra speed for the cost differential. You are better off with an after market cooler rather than the ones that come with the processor (retail versions ship with a standard cooler).
If you don't know why you need an i7, go for an i5. It is better value for money. I have an i7, if I was buying again tomorrow, I'd go with an i5...
Power supply: I tend to build my systems with a 650W PSU. I don't do any gaming.
Depends what you mean by "light gaming", but I've never seen the need to spend more than ~£70 on a graphics card (and that was £20 more as I wanted a passively cooled card).
Go with the Z77 motherboard with the features you want on it (number of SATA ports being the key one - it's surprising how easily these get used).
If you have an old SSD, it is likely to be a pretty poor performer compared to current generation technology. Crucial M4 and Samsung 830s seem to get the nod here.
Power supply: I tend to over spec these (having had a couple go on me) and try for 80 plus gold or better. My PCs (I have 4 that run proper PC supplies and a few running on laptop supplies) are on 24/7 so efficiency is important. Also the higher efficiency supplies tend to be quieter so... I've found this page:
http://www.realhardtechx.com/index_archivos/Page541.htm useful as a place to look for reviews of power supplies and this site:
http://www.silentpcreview.com/Recommended_PSUs very useful for comparing PSUs from a noise viewpoint (as well as coolers & gfx cards - although "passive" in the description is normally a good indication
).
Case is personal preference, but I reuse my cases over time, so I have invested in the ones I bought. I can recommend the Antec P183 v3 (I have the 182 case and love it) or the Fractal Design Design cases (I use for a fileserver as it has loads of disk trays). Not cheap, but to get quiet cases needs some engineering and deadening...
Hope that helps.
