As Chris says it's a fairly easy 3 maybe 4 hour drive (depending traffic) down the turnpike.
I like to stay in the historic South Beach area of Miami. My favourite hotel down there is
http://www.richmondhotel.com/ , The Richmond has a great location,decor, facilities and an excellent staff. It is a family run business and I thought they all worked hard to provide that little bit extra. I picked this up on
http://www.hotwire.com and got a great deal on the room. I think I payed about $60 a night when standard "rackrate" was more than $150. If you decide to use hotwire, be aware you don't know the name of the hotel until AFTER you commit to the booking. Also in this area (South Beach) there are a lot of "boutique" hotels that only cater for 2 people per room so do read the info on the hotel BEFORE you commit to a booking if you're group is more than 2 people per room required.
With Hotwire I usually stick with 3* or above and haven't been disapointed with what they have provided. Do be aware that along the beaches in Miami you will often be charged a "resort fee" on top of your headline room rate, for use of the pool/beach (even if you don't use them) as well as be charged for parking. Places to park on South Beach are at a premium. I think I payed about $8 a night for parking ($12-16) if you want it valet parked. Alternatively you can park in one of the "municipal" car parks (there are many) yourself (which is generally where the valets put them) for about $6 a night. For one or 2 nights I'd valet park it to save the agg of finding the damn car and having to stress about getting your luggage in/out of the hotel. For a longer trip I'd probably valet on my way in, check out the nearest parking lot and use that for the rest of the trip.
There are some great hotels in this area, I'm not sure what sort of price you're looking at but here's a link to just some of the hotels in that area.
http://www.southbeach-usa.com/hotels/south-beach-hotels.htm
Lowes , Delano and Tides are all upmarket hotels and wonderful to look at ( but a bit pricey). I've also stayed in the Riande Continental (decent chain hotel, convenient location) although I had a row with a particularly stroppy check out clerk there. I've stayed on Ocean boulevard a couple of times, VERY convenient, but it was a little noisy one time as that stretch is very popular with party goers. I prefer to be on Collins where you are just a short walk away, but it's not as noisy.
