WWW, do you think she'll actually want to use it straight away? I know with my DSs I didn't use a sleeping bag when they were newborns.
If she might not want to start using it until the baby is a few months older, I'd get a slightly heavier one.
Or you could get the sort where you have an inner and outer lining which can be zipped togehter to make a warm one for the winter - I have one but can't for the life of me think who makes them! I don't think it's Grobag though.
You can get the double-layer one from Urchin. Quite a good idea if they're not going to start using it until the summer's over because it's far too heavy-weight for summer, IMO. Otherwise get the lightest possible. I don't know what tog, but it should be one, or at most two, layers of fabric with no wadding or padding between the layers. Buy one which will fit up to 6m and they will still be able to use it in winter, with warmer babygro underneath and a blanket on top if necessary.
I'd go for a 1.0 tog -- probably too heavy for the height of summer, but a good general late spring/summer/early autumn weight. DS was always in 0.5 tog when it was really hot, but who knows what this summer will bring.