I just recently journeyed down the same path.
After much deliberation and trips to the local music stores, I settled on
the Alesis QS 7.1. Granted, I'm certainly not a synth expert...Hell this is actually my first since I'm a guitarist....but the Alesis products sounded great to my ears.
IMHO, they are built very well, and the 7.1 (76 key)was about $450 less than the price of the Roland XP-60 (61 Key).
Honestly, most of the other keyboards I tried felt...well...cheaply built compared to the Alesis.
I am trying to lay down some Stabbing Westward/Chemlab/God Lives Underwater type stuff, and I think I can squeeze what I need to from the Alesis. As for you wanting to do hip-hop and rap, if you want more than what the base QS mixes will give you, they have Q-Cards to fill the gap...AND one just happens to be for the rap/hip-hop genre.
With all the stock programs and mixes in addition to what you program yourself, it should keep you quite busy.
The only drawback to the Alesis keyboards is the lack of a built in sequencer like the one that comes on
the Roland XP-60 and 80, but thats what a PC is for, right?
My advice to you is to find a local music store where you can have the models that interest you in a side-by-side comparison. Find out what sounds and feels good to you and go for it.
And on a related note, I found the best price on my QS 7.1 at
www.8thstreet.com. I paid $949 (free ground shipping) as opposed to $979 plus sales tax locally.
Good luck,
B.L. Zeebub