Two factors to consider.

One, a cab will cost you more than the rental car by the time you factor idle time. (I used to work north of DFW about four miles west of Frys - or about half the distance you will go in the cab) and cab would routinely run me $20 to $25 one way to drop me at my building.

Two, time - rental car pick up/drop off is not uber-convenient and will add a good hour to your overall time vs. hopping in a cab.

That said, with five hours, I would rent the car so I'm not paying for someone to idle outside of Frys (and calling a cab to come back and pick you up could have you waiting 30 or more minutes - we just don't do cabs much in Dallas).

Besides, I suspect you will want to take your time and enjoy that Frys - it is one of the nicest in the whole chain. If you see the 30-ish young redhead and brunette wandering around the store together, wave at your fellow FTers - that would be PeachesBB and I - we've been in there several times a week over the past couple of months and know a lot of the salespeople in the various departments by name.

(Also, if you need to eat, this Fry's has a cafe location - coffee drinks, gelato, and basic salads/sandwiches. If they are pulling an order for you, you can go eat and they'll let you know when your computer order is ready.)