I agree with LunkHead's advice about trying it out for the two weeks, and I am sure that you will fall in love with it.
Email on a BB can not be trumped. It is the best around bar none.
Jivetalk provides a great IM experience, but for your part time needs as you stated the built in clients should work just fine.
As for the camera, there are 3 size settings, as well as 3 quality settings.
Large (1600 x 1200)
Medium (1024 x 768)
Small (640 x 480)
Taking smaple pictures of the same item with the same lighting, the largest file size was 602kb (large & superfine) and the smallest was 22kb (small & normal). I sent an email containing 8 images with a total size of 2946kb so I do not believe you need to worry about the limit. It did take a decent amount of time to send (several minutes) but it made it flawlessly through to its destination.
Video recording for the curve has been confirmed by a few sources if I am correct with an upcoming OS update, and if you plan to use the camera, video or other media features of the phone buying a micro sd memory card is greatly advised.
You will get used to the smaller keyboard and I personally find that my 8320 if very easily used one handed. Autotext is included, and syncs when you back up your device, so there should be no fear of losing your personlized text shortcuts.
BB's calender and to do lists are integrated, and sync with Outlook or several other programs. There are also many software suites that offer document editing, everything from word docs to spreadsheets.
Sorry about the last question as I have no experience nor knowledge regarding the use of a blackberry and yahoo briefcase.
Hope that helps with your decision, and I am fairly sure a new user will be joining those of us that are addicted!