I think you're being a wee bit ambitious looking for 0c in a 500g bag. I'm not aware of any that quite meet that, although some come quite close. Take a look here at the PHD Minim range - absolute top quality down bags and stripped down to the essentials (I really rate them, I have 4!!) (Peter Hutchinson is the guy who originally founded Mountain Equipment years ago - he's a down guru!!)
The Minimus stays within your weight limit but is only rated +5c. That said I've used mine down to 0c, even to -3c or -4c a couple of times but only with clothes on inside, and that probably doesn't meet your 'comfort' criteria.
The Minim 400 is more than your rating. PHD tend to do a sale about twice a year, and in the last couple they've done a 'special', a Minim 300, rated to 0c and weighing about 580g. I absolutely love mine. Thinking about it, at 5' 6" you'd get away with their short length which would bring the wight down a bit. You could explore the idea of a short Minim 300 using PHD's 'design your own' service on the website (base it on a Minimus or a Minim 400 and then adjust down-fill, fabrics, length to suit) - the only problem with that is the price of the D.Y.O. bags, it's be a lot cheaper to wait to see if the Minim 300 appears in the next sale, but that's not likely to be until February.....
All Matt says about PHD seems very sound (I've not tried their bags personally, but the reputation is excellent).
As an alternative, I have been paying close attention to Valandré that are made in the French Pyrenees and are gaining an excellent reputation - especially in the US lightweight backpacking market. The Mirage might be worth seeking out as an alternative to the Minim 400. I think they look fantastic:
Indeed - Cumulus is another good brand I failed to mention!
As with all outdoor gear, the key is to take a good look and try getting into the sleeping bags before you buy them (or at least order one with a decent return policy). You can never really tell whether it will be warm enough for your purposes until you find yourself in the real situation. There's so many other factors beyond simply the insulation of the bag to consider (level of fatigue, level of nutrition, age, basal metabolic rate, atmospheric humidity etc etc).
The fit of the Valandré bags is unusual - relatively wide around the torso and narrows dramatically around the lower leg. Some find this very effective - some restrictive; you'll have to judge for yourself.
The closest I've been using to what you're after is the Mountain Equipment Xero 250. It's got a 3/4 zip though.
Very light and compressible, and surprisingly warm. Not 0-rated by the EN standard, but I've used it into single figures and been fine. They're great bags, super comfy with the stretch seams on the top.
I think I'd have to agree with ptc*. the best lightweight bag you'll get your hands on easily will be the ME Xero 250, however, even this doesn't quite meet your needs as it's got a comfort rating of 8C and weighs in at 560g.
A couple of other option would be to get the Xero 350 which is about 150g heavier but does go down to about +5C or to get the 250 and just invest in a decent silk liner that would obviously compact down very well but offer an added 5 degrees of insulation needed.
