since the power was out.. ( i assume the power is out before your basement was flooded ) yes it,ll boot up..
1. rinse for the 1st time, dry it, spray all the contacts on the board with contact spray, dry it, rinse it again, dry it, put everything back.
2. no the CMOS battery would not give u any problem
3. parts to worry about : CDROM,HDD ( these parts contain moving parts usually flood water carries mud and dirt that might damage these moving parts )
if u know how to disassemble a CDROM drive ( or any optical drive ) without destroying it. then i suggest to disassemble it and clean everything ( except for the optics ) and a HDD can be quite tricky some HDD had holes for no apparent reason and some dirt or mud could be stuck in it , i suggest to check your drive(s) thoroughly.
done this on a system that was kicked by my drunk-friend into a swimming pool at a LANparty .

i had to dry it for three days just to make sure it's dry... unfortunately the HDD died
which reminds me not to serve BEER on the next LANparty at my house
