Would you suggest buying SX30 anyway since I'm a rookie when it comes to DSLRs or G12 would be better?
I'm afraid we are confusing you. Nothing has been said here that is incorrect, but I think it might help to step back a bit.
First, think about what you want to take pictures of, what your picture taking habits are and if those habits are likely to change. From what you've written, it sounds to me like you are interested in a "step-up" camera, but don't want to spend a lot of money.
It also sounds like you may not be particularly interested in lugging around a camera bag with three or four additional lenses and switching them out depending on what you are photographing.
What do you enjoy taking pictures of now? And, what do you think you will want to take pictures of in the future?
The SX30 sounds like a good camera for someone who enjoys birds, zoos and wildlife and wants to be able to zoom in as close as possible. The G12 is a great all-around camera for carrying around and being prepared to take a picture whenever one presents itself to you. If you want to take pictures of family, vacation pictures of scenery, cities, historic sites, etc., pictures of just interesting things you see on the street, the G12 is a good option.
The G12 is small, unobtrusive and the top of the Powershot line. The sensor is slightly larger than the SX30, it can produce high quality enlargements (within reason) and has many professional features.
There is a saying that "the best camera is the camera you have with you." You can't take pictures if you don't carry the camera around with you. If you're likely to view a DSLR as a hassle and a burden, then you are much better off with a "point-and-shoot." (As an aside, I find the phrase "point and shoot" a bit snobbish and frankly inaccurate. Almost any DSLR can be a "point and shoot" in fully automatic mode and cameras like the G12 have a full range of features that allow photographers to use them creatively.)
It's true that for not much more money you can buy a Digital Single Lens Reflex (DSLR), but you will be buying the bottom of the line DSLR vs. top of the line Powershot. That may be the right choice, but it may not.
The whole discussion of f-stops is confusing. It's not really fair to equate an f2.7 lens on the Powershot with a f15 lens on a Rebel. F-stops are format neutral in terms of exposure. An exposure that requires f5.6 at 1/125 of a second, is going to be the same no matter what camera and what format you are using.
I do not have the technical knowledge that some others on this forum have, but from what I have read here it sounds as though a small format camera will have greater apparent depth of field (the distance at which various objects will be in focus) at comparable f-stops. You might think that is an advantage, but it can be a disadvantage as well. Sometimes you want to separate the main subject from the background and that becomes harder with a small sensor and the greater apparent depth of field.
Something else to consider with the SX30 is that even with image stabilization, the longer a lens, the harder it is to hold it steady. Image stabilization helps, but zoomed out to 840 mm, you'll still need a fairly fast shutter speed to avoid shaky pictures.
Bottom line: go to your favorite electronic superstore and play with the cameras. Think about what you are looking for in a camera and how much you want to invest. Most DSLRs get updated every couple of years (some more frequently, some less) and the whole camera industry is undergoing rapid change, so any purchase you make today is likely to get replaced in a few years. If you are not planning on buying additional lenses and don't want to spend a lot, you may be better off with either the SX30, the G12 or another upper level "point and shoot," even if you eventually decide to take the plunge with a DSLR.
