You just gotta wait and see what you get, take it from there. If you have an at system, then you likely have an at psu, incompatible with an atx mobo. You likely have edo or similar memory, again incompatible with modern atx mobos. You may get a s370 processor and a slot1 mobo, you'll need an adapter. Then there is the case....and the device drivers for the vid card(the old system may have onboard video, it was common) and the sound card(same issues)
Upgrading can be tricky, parts selection is an artform, and varies depending on how deep your pockets are. The top link in imaginers post is to a decent little board, one of the local gurus (read BS artist) uses them for cheap upgrades. They actually work rather well.