Before getting a new seatpost I'd try these first:

-run your tires 5 to 10 lbs less pressure on rides
-try a premium set of bike shorts with the top grade internal pads
-for a big difference in ride go test a friends or demo a set from a shop of larger tires/better wheelset

FWIW - I've noticed a big difference in ride between two similar sets of rims that I have in 18/24 and 24/24 spoke count.