A sleeping bag is a really important bit of kit and like a lot of folk starting out in the outdoors I've bought some really bad ones when I was starting out ( anyone remember the Argos 'Moon bag' one with Millium silver heat reflecting lining).
was just typing about this. Coldest night ever spent outside, and this was billed as a 3 season NASA bag, so I really agree about the rating system making no sense. Also the blurb was a load of rubbish, so be careful. I think you have to ignore the rating system, and look for the tog rating(I think), but not sure how it works for hiking bags. This system works, it rocks in fact, if you do not have to carry them.
Like a lot of things you have to make trade offs as well. I love air beds, best nights sleep I ever get, but I would not carry one. The camping sleeping bag I have is fantastic, but it is simply to big to hike with.
The big trick is to start off as clothed as possible in your mummy bag, and slowly undress as you warm up. You want to sleep in as little as possible, but you are the heater for your bag so it makes sense not to start cold.
The best bag I have ever been in was an English army down mummy bag(Seventies, or Sixties), never slept with the zip done up. The problem is it was heavy, and apprently a nightmare if wet.
If money is the issue, I would be tempted to walk in to Millets(it always a good idea, to keep an eye out at the camping stores - argos have had some mad deals recently), and see what they have on offer. Have a feel, see what strikes your fancy. Nothing, go back again next week. Keep an eye out. If you find one you like, but the price is wrong, hit the net.
The point is to get out there and be a little cold for a few nights. Listen to the advice, and work out what you did wrong. I still use a Pro-Action sleeping bag(Bought in a hurry years ago to replace the 'Moon Bag'), have done for years, I am thinking of replacing it, and to be honest, will end up with a Coleman.