AF_Roadie,

A suggestion is to borrow an el-cheapo bike from a buddy.. or even pay a visit to the Salvation Army store to see what's in...
For that "beater" bike, try and find something AS CHEAP as possible... the more money you save here, the more you can spend on your new roadie...
Sadly, bikes are quite expensive... but some important things are yes, the saddle...
So find one that's real squishy/gelly to give you that comfort and plant it on the beater... it'll make a world of difference [i've tried...]
Pay a visit to the under-700 thread to get some ideas for what bike you would like next...
Link
Good luck with your purchase

-Peter