You get much more value for money upgrading to some decent strings (eg. Dominant) and a decent bow, rather than upgrading the actual instrument.
I have a Michael Poller violin (only £180 new) strung with Dominant strings. It has a beautiful tone and is so easy to play, it almost plays itself. Andreas Zeller instruments are also very good value for money.
I recommend changing the bow first if you've bought a cheap instrument outfit. The bows supplied are usually terrible, and really don't help if you're learning. Often, the balance is wrong, they aren't straight, and they don't have the right curvature. Factory strings should be replaced straight away too. Why make things harder for yourself if you're only just beginning.
I 've used both wooden and carbon fibre bows to play cello, violin and viola. Although I have no complaints regarding the carbon fibre or wooden ones, I think it is just down to personal preference which is better.
Carbon fibre does have its points, such as not warping... But there does seem to be a subtle quality missing when I compare them to a wooden bow. Maybe it's the resonance of the wood.