Briarbow is right if you get a 0 degree bag and tend to use it in the mid 30s you will roast in general. Roasting in a synthetic bag will tend to not be as hot and sweaty for the synthetics I have used tend to wick a little better. But everyone sleeps different.
A liner can help and offers another layer if the mercury drops slightly beyond the rating. But many bags dont actually work at what they are rated at. If you want a liner sea to summit has a wide variety out there both silk and thermolite. The thermolite will add the most amount of heat but it is a little bigger to pack...by no means is it huge. But to me a good sleeping pad can help so much!! So as I mentioned make sure if you sleep on the ground you get a good sleeping pad. One area a where a bag can lose its warmth can be the wind. Wind can really get you and that is where layer can help or a bivy but this entire subject gets even more complicated. If you sleep in a tent, nevermind this subject or debate.
One suggestion, go to an REI or some outfitter and see if you can get into these bags as well as on a sleep pad. As I said, I love the cats meow a lot but it was narrow inside and that made it a complete deal breaker! So try before you buy, check the zippers, adjust the mummy hood (Most of the time I dont use them), move around in them, and use the zipper to get in and out. Also...take your time looking but remember one bag cant do it all but you can get one that will cover most of your outings.
Oh and JGON...that is a sweet bag!!! However it is nearly double the specified budget, I looked at it as well when I was considering Down it looks tight but was actually not bad absolute awesome bag! But when you get into the 300 range there are a ton of great bags! I really like the Marmot Sawtooth for a down bag...you can get them around 200 now. But I also like the Mountain Hardwear Phantom as well. They pack around 2lbs...10-12 oz lighter then the marmots. Then again if I were to buy one down bag and only one...western mountaineering is my pick! Just BIG CASH!
Carspidey, let us know what you do but honestly the Cats Meow will serve you well and will not break the bank! It is a well rounded bag that many have used and loved for so many years...it has been around for ages with good reason...it works well! Just make sure you get in it to make sure it isnt too confining for you. Good Luck!