Newbie asking questions!

I am in the process of building my first two machines at the same time. I am
planning to build afresh but reuse an two old cases and PSU's. I have found
that my ATX AOpen AMD motherboards I purcahsed (to go with the AMD 2500 and
2600 processors I got) dont physically fit the cases, and one of the PSU's
doesnt fit into the motherbaord connector either (wrong connector at the end)
Am I doing something wrong as the cases I am reusing are not that old (around
late 1999) didnt think building of motherbaords would change so much in that
time. And do I just need an adapter for the PSU or is it a bin job.

By the way can someone clear this up.....why are some PSU's a different wattage
to others. I want to run the usual plus two hard drives and a DVD ROM I wouldnt
need an uprated PSU for this surely. Also with themral compuind do you only get
enough appliacation in the syringes for one CPU/Heatsink or will I be able to
squeeze two out of it.

Many Thanks for everyones help