Second, this shounds like a depth of field problem. Higher apertures (f-stops) have more depth of field. Use the highest aperture that you can.

The apertures are: F1, F1.4, F2, F2.8, F4, F5.6, F8, F11, F16, F22, F32, F45, F64, F90, etc.
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By the way, f/22 is not neccessary for group shots, only when you want both the background and foreground to be in perfect focus. What you want is to still be able to separate your group from the background, for less compositional clutter but still retain a DoF long enough to have focus on one linear plane at a specific distance from the camera. You stop down to f/22 and the background will distract from your subjects and it will look more like a snapshot than a nice group photograph. That’s my opinion.
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Stop the lens down. If you want everything in focus put the lens at f/22, and make sure the shutter is above 1/125.

That should solve your problem. If it doesn’t you need to get a new lens…seriously because that ones broke.
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I’d suggest getting the book "Understanding Exposure" by Bryan Peterson. It will explain everything you need to know about exposure and foucs, in easy to understand language.

For your group shots, I’d suggest using a setting somewhere between f11 and f16.. use M mode and adjust the shutter speed til the exposure is correct. Use your flash and see what your results are.
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A sharpening solution in PS might be a high pass filter overlay of about 3.
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Another problem may be that your lens has a backfocus or forward focus problem. Backfocus is the most common, but both can occur. It’s rare with Canon lenses, but it can occur with them just like it can with any other lens. It is much more common with third party lenses like Tamron, Sigma and to some extent Tokina (all of which make some excellent lenses). The only thing that can be done if this is the case is to send it in for service. All of the companies are pretty good about correcting this problem. You can lookup backfocus using google and decide from that if you have a backfocus problem.

The 30D should give you a higher quality image than the XT’s since it’s basically a 20D with some bells and whistles added. Unless you are doing something wrong, it sounds like your focussing system is out of alignment if it’s not a matter of sharpening the image.
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Make do and carry on if you can.
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Don’t skimp on the tripod a good tripod and head pays for itself many times over and will last you a lifetime. You may want to add a grad to the list of camera filters. And if this is your first DSLR then the digital field guide for your camera by wiley may prove easier to read digest than the manual that comes with the camera

But in truth what I carry would be different if I were doing mostly wildlife work or macro work and even within areas of photography many folks have different styles. As an example many portrait photographers are shooting with wider angle lenses than would have been considered a "standard" portrait length a few years ago as that is a look they like better. So you may want to get the camera and a few necessities then wait and start shooting. Buy the other things as you find you need or want them for a particularl look. If there are photoclubs in your area you may find folks there that also shoot nikon that are willing to let you try some of thier gear on group events to see if its something you need or want.

hope this helps
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