Power cycling the router will not change the previously stored configuration, and based on the version wrt54g router (there are well over a dozen versions of this router), this can be found on the bottom of the unit) it may not automatically update. The IP address that you see in the router may well be from your last internet service.
*The version wrt54g is definitely important, as capabilities and set up routines have changed based on version over the years.
I recommend using the rest button (usually a small recessed button that you hold down with a pen tip while the router is powered) and setting it up from scratch.
If you are not comfortable setting up the router via its embedded web page, utilize the installation software that came with the router. If you do not have it, it is easy enough to go to the linksys support site:
http://homesupport.cisco.com/en-us/support/routers/WRT54G-TM/download
Enter the version, and available downloads including installation software, firmware updates, user guides, etc. will be shown.
By the way, while doing this, I would recommend ensuring that you have the most current firmware installed.
As your router worked with your previous internet service, there should be little difficulty using it with comcast.
BTW, not trying to sell you a router...but, the new EA series of Linksys routers (they have just hit the stores) are incredibly easy to set up compared to previous generations. I could get my 82 year old mom to set up one of these with Cisco's easy-connect, and she knows nothing about networking. They do not even need to be wired to your computer during set-up.
