Hi
A 3 season bag in summer when hiking with a load will be too much unnecessary weight I think
Sleeping bag technology has moved on leaps and bounds recently and is worth researching
I got a Nanok cool
http://www.nanok.no/engelsk/page28/page28.html from Lakeland Bushcraft which was a genuine bargain(Thanks Lurch)
That bag does me from early May till September in UK I use a lightweight bivvy bag with it and can add a liner if needed
The main thing is it weighs 880g!!! I have used it in winter on Exmoor with a fleece liner and been fine as well
Take a look at some of the new bags from Vango, Highlander and Berghaus
In particular one of the Highlander synthetics claims a warmth to weight ratio very close to Down
Army issue bags are not really good for hiking but The Jungle bag is probably best for Summer
Its worth researching what you are buying I think there are some bargains to be had when you look at top of the range stuff and what you are actually getting for your money Synthetics are catching up with Down and they have some real advantages
Also get a good quality sleeping mat In this area Dutch army surplus excel but Multimats are good as well again research
Of course being a gear freakI would love a RAB down bag but in all honesty can i justify spending that on a bag for UK use (well i can to myself ;but not to SWMBO)
My apologies to Richard if this has all been discussed before but I think some of us who are a bit more experienced
and have tried different gear can try and help out other members without being too terse