You're going to have to decide what's best. If you plan on doing nothing but highway driving, you can probably get away with putting off the 3G alternator for a little while. If you're going to be doing a lot of city driving, you might want to go with the alternator upgrade before the new radiator (assuming your current radiator is in good shape).
The stock alternator only puts out 20 - 25 amps at idle. If you're stop-and-go driving on a hot night, with the lights on, your HVAC fan on, and your e-fan running constantly, you'll be about 60 amps in the hole.