Thinking of Upgrading from Canon Rebel XS (1000D) to a Canon Rebel T3i (600D) or T4i

Hi, everyone. I've actually lurked around this site for quite a while, but decided to join finally. I'm a hobbyist photographer, most of my photos are of my kids (4 year old and 7 month old girls) and various family photos too. I enjoy using ambient light whenever possible so my Canon 50mm f1.8 is on there probably 90% of the time, I also have the Canon 18-55 IS kit lens. Good low noise high-ish ISO performance is also a need of mine. I've really enjoyed my XS, but I only feel comfortable with the quality of ISO 400 or below. I will go to 800, but that is a bit too noisy for my tastes. I was hoping that the T3i would be an improvement. Ideally it would be nice to have the ability to take up to 1600 ISO with acceptable noise. In essence I guess what I'm asking is the T3i enough of an upgrade noise wise, considering the sensor packs a lot more megapixels in there. I'd also enjoy the superior metering it would provide compared the the XS.

I see that Amazon is offering a great package deal right now for the T3i. Basically I can get the T3i with kit lens, Canon 55-250 IS lens, SanDisk Extreme 16 GB SD card, Canon PIXMA Pro9000 Mark II Inkjet Photo Printer, Canon Photo Paper, and a Tiffen 58mm UV Protection Filter for $650 after $400 rebate. I'd sell my XS, one kit lens, the printer and the paper on either CL or Ebay. Hopefully after all that I'd only be paying somewhere around $100. Not a bad upgrade price, plus the zoom lens I've always wanted to try out.

I'm also considering the T4i, mainly due to the 9 cross type auto focus points as compared to the one center cross type in both the XS and T3i. But I'm not sure that's worth an extra $150.

I want to get a better standard zoom lens too. The 50mm is great, but indoors (where it's the most useful) it can be challenging to get the whole scene in frame. I've been eyeballing the Sigma 17-50mm f/2.8 EX DC OS and the Tamron AF 17-50mm F/2.8 XR Di-II LD non VC. Is the $150 extra for the Sigma worth it for the image stabilization? I have it on my kit lens, but haven't really tested it without the stabilization active. The Tamron stabilized lens doesn't have a great rep, so I'd opt for the non stabilized lens from them. Both seem like great lenses, but I've heard there can be focus issues with the Sigma.

I will eventually probably get one of these lenses, but thought if I could upgrade the body it might be worth it. If I reach my goal of $100 total after sales for the T3i I actually have enough saved up to get the Tamron lens right now. But I want to buy for the long haul, not what I can just get today. Of course what I want for everything may not be realistic either.

I look forward to your input. Thanks in advance.