Larry's post is great, and I just want to emphasize one part:
With 2 weeks to travel, my recommendation would be to pick one of the countries, then have stays in 3 or 4 places, with additional day trips perhaps.
If you want your birthday to be a celebration instead of an endurance test, please follow this advice!
If you go on an escorted tour, there are two week tours that take you to both Spain and Portugal. However, if you're doing it on your own, be aware that connections between Spain and Portugal are surprisingly skimpy. There are a few trains (often in the middle of the night), there are some buses, and there are flights. A car rented in Spain and dropped in Portugal, or vice versa, can have very high drop charges.
Connecting Greece and either Spain or Portugal will not only involve flights, but depending on where you want to go, would most likely involve connecting flights - meaning most of a whole day lost getting from one to another.
Summary: Pick one country, or at most two, and see it well, rather than trying to see a bit of all three and just being exhausted and frustrated.