1) Wait until you see the "channel lineup" change occur on your normal TV service (as announced in a postcard sent by Comcast).  Comcast will not even take an HSI order until then.

2) Decide how you want to get your cable modem (the connection between the cable and your PC).

        a) Have Comcast supply it and charge you a monthly fee.

        b) Have Comcast sell it to you (about $130).

        c) Go to Radio Shack and buy an RCA modem for about $80.  If done before June 30, you will get an $80 rebate (making the modem FREE).  Radio Shack will call up Comcast while you are there at the store and schedule the appropriate installation for you.  You also get 3 months of HSI for only $20 per month.  This will save you about $70. 
In total, this is the BEST DEAL - saving about $200!!
 

3) For those of you who want networking in the house, also consider going to Circuit City for a slightly wider selection of modems, wireless, hubs, etc.  But the Radio Shack deal is the best (even if you don't use their modem).