I've booked in may at Grand President Hotel for 1700 baht. Looks like a nice place.
http://www.grandpresident.com/rooms.html
I think its on Soi 11
Taipan Hotel on Soi 23 i think was the same price. Nice facilities.
Plenty of choice on Suk
Good luck
Samui is great fun and a pretty place. I'd give it 5 days..
Hire a scooter and go around the island.
There is a bunch of places to book dive trips and day trips.
Pumping nightlife and plenty of p4p.
Enjoy!