Actually, being so much closer to the equator than we are up here in the US, the temperatures in CR do not vary that much. In Jaco, the typical temperature stays pretty much in the low to mid 80F range during the day. Rainfall and hence relative humidity are a whole other story and can make the perceived temperature seem much higher (aka the Heat Index). Ironically, it can actually be much cooler at night during the rainy season.
Here is another interesting link for weather in many different areas of CR for each month of the year.
http://costa-rica-guide.com/Weather/WeatherMap.html (be sure to check out the seperate page for temperature differences). That site has a cool interface, but for those who can't figure it out here are direct links to the Jaco rain and temp charts
http://costa-rica-guide.com/Weather/images/JacoRain.gif
http://costa-rica-guide.com/Weather/images/JacoTemp.gif
If you notice the average temps in Jaco in July are actually slightly lower than they are in April and rainfall, though still higher than during the dry season, actually drop significantly during July and August. This phenomenon is known as the veranillo or the little spring.
I'll leave you with one last link.
http://www.weatherimages.org/data/heatindex.html This helps you figure out the heat index if you have the air temp and relative humidity. We have an estimated temp of 85F but the charts only go down to 90F, so we have to use the calculator that you'll find way down at the bottom of the page. We don't have the exact relative humidity amounts, so I tried plugging in different figures. At 85F and 50%RH the apparent temp seems to rise only 1 degree to 86F. However at 75%RH the temperature will seem more like 95F and at 100%RH it seems like a sweltering 107F. But thats during the height of the day by nightfall it has usually rained, dropping the RH significantly as well as the temps.
Thats still way too hot for me. I'd be afraid to go out on the beach during the day as I might melt. I'd stay in SJ or the mountains if I were you. Besides, where did you hear that "the Beatle Bar action/choices are better than the BM in San jose". I've never heard that.