So, my wife is turning 30 in a couple weeks (Yeh...I love being married to a younger woman). I bought her the following for her birthday:
First off, B&H is usually your first and last stop for photo gear. Check other places, but 90% of the time or more, I end up back at B&H.
You've just about got everything you need. You DEF don't need additional lens capability. With the 2x teleconverter, your 75-300 gives you all the way to 900mm equivalent focal length lens (1.6x crop body x 300 mm = 480mm...double that to 960!!!)
The 75-300 will be fine for flower shots.
There are only two, or possibly three things I can think of to add.
1) Third party books on operating the XSI. They're actually pretty good and you can find them at Barnes and Nobles for $29.95. They supplement the operators manual with helpful tips quite well. The DSLR menus and all of the functions can be very overwhelming to a beginnner, so the book helps to sort through all of the functions of the camera and once she reads it once or twice, the functions become far less intimidating.
2) Close-up rings. This may or may not be attractive to her; however, since you said she likes to photograph flowers, these might come in handy. They're cheap on ebay if you buy the kind where outofocusing isn't transmitted from lens to camera...not a problem in macro photography where most focusing is manual anyway. Still moderately priced even if you buy the version that retains autofocus through the rings. I'd say that you should be able to plenty of flower photography with the 75-300, but the rings would enable you to increase your range (to photograph things on a smaller and smaller scale) beyond the natrual limitations of the lens.
3) Software. Pick up a copy of Photoshop CS3 and Lightroom. Photoshop is for image enhancement, Lightroom for image catloguing and maintenance.