The old Minolta lens might work on the Sonys. I'm not sure when the Minolta mount changed. There's also the factor of how good of a lens it is - if it's a basic kit lens it probably isn't that advantageous to use it as a deciding factor in what system to choose. If it's a high-end lens (like a 70-200 f/2.8 constant aperture zoom, typically worth $600+) then it might be a contributor to your decision.
The Canon DSLRs are decent. The XSi is a HUGE improvement to Canon's lowend compared to the XT/XTi (which, absurdly, lacked spot metering in a $600 camera, which killed the deal for me back when I was camera shopping and went with Pentax instead. Pentax has sadly lost a lot of their value lead in the sub-1000 price range since then.)
For indoor photography you're going to want to consider a fast telephoto lens (something like, for example, a 70-200/2.8 like I mentioned above, where the lens could easily be as much as the body.) The nice thing about DSLRs is that you can start with some basic lenses and then upgrade as you see where your photography needs take you.
Each camera system has something to offer, and it can be quite difficult to buy into a "system".
Canon and Nikon - These are the "big boys" and dominate the professional market. Lots of accessories and glass (lenses) available, but their market dominance means they often command a price premium, especially on glass.
Olympus - They are known for having some of the smallest DSLRs out there. This is partly because they use a smaller sensor than anyone else, which can limit image quality and noise performance.
Pentax - Previously a leader in the camera market, they're playing catch-up now. 3 years ago they had HUGE amounts of promise with the K10D which provided incredible "bang for the buck", the K20D was not as much of a value leader (much more expensive than the K10), and they haven't announced any new models recently in a rapidly evolving market. I'm hoping they make a comeback as I'm invested into their system, but it's looking somewhat bleak now.
Sony - Relatively new to the DSLR market, it's clear they mean business due to their purchase of Konica-Minolta to get a headstart. Their DSLRs carry some serious promise and if I were shopping for a new camera now I'd seriously consider them, despite my idealistic hatred for Sony in general. If they have any DSLRs that don't use Memory Stick (why does Sony STILL insist on a standard no other company uses? Their insistence on starting format wars is the main reason I hate them.), I would advise giving them some serious consideration.
I'd strongly reccommend reading into reviews for bodies in each system within your price range, and also reviews of glass within your price range. For example, one of the reasons the Pentax K10D sold so well is not just because it was a good body, but because it was paired with a kit lens that provided unusually good performance for a lens in its price range. A lot of the Canon and Nikon standard kit lenses have pretty bad reputations, while even the first generation Pentax 18-55 was highly regarded despite its low price.
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