Is it worth upgrading my Lumix G10 to a Sony Alpha a33?

Currently am shooting with a Lumix G10 then i've owned for about 6months, it the camera has been a slight disappointment to me, as i didn't realise at the time of purchase it didn't have an in body image stabilisation which means my choice of lenses is minimal. Also due to the fact it is a m4/3 lenses are very expensive.
Is it worth upgrading to a Sony Alpha a33?
So i will have a much wider choice of lens, and all accessories will be cheaper?
Or should i upgrade to the the nikon D300?

With the four-thirds mount, you are not limited to getting Lumix or Panasonic lenses. You can use other brands also. Yes, the better ones do cost much more like those from Leica. The Sony alpha lens lineup isn't that much wide either even if you include those from Carl Zeiss. If you know exactly what you need and know exactly what you're doing, you can survive with that small lens lineup. Pentax has a small lens lineup too if you look at the digital models but since all lenses made by and for Pentax is compatible to a great degree with their new dSLRs, the choices suddenly open up wide.

4/3rds lenses are expensive and thin on the ground. The Sony has a much wider range and takes all the old Minolta 35mm AF lenses as well. Don't forget that you have to multiply by 1.5 to get their effective equivalent though. A 28-80 will act like a 42-120 (35mm equivalent) on a Sony A33. The Sony has a physically bigger sensor than the Lumix which gives better images. The A33 has a fixed mirror too, so you don't get the blackout effect in the viewfinder when you take a shot.

No point. You'll just waste money on the upgrade, the point of a CSC is that it's compact.alpha lenses are all rather large.

There are some very interesting lenses that are available on MFT. Leica M-mount (some of the best lenses ever made) and M39 lenses from rangefinders are compatible via adapter, you can adapt, bigger four thirds lenses too. As well as pretty much any SLR lens with manual aperture.

You should go with Nikon D3000 Digital SLR Camera (18-55 mm VR Lens Kit)

10.2M Nikon DX format CCD sensor
3 inch hi-res LCD monitor
New Guide Mode for easy operation and enhanced picture taking skills
11-point autofocus system for tack sharp results
Active D-lighting for shooting in high contrast conditions