Clover is a sign of nitrogen poor soil. Fertilize with a good organic fertilizer and your good grass will have the nitrogen it needs to crowd out the clover instead of the other way around.
Check out this site, it spells it out pretty well.
Have you seen any clover yet this year? It is probably too late, but if you put down corn meal gluten in early February it will keep the seeds from germinating in the Spring (just don't use if you are seeding a new lawn) and it is a good source of nitrogen.
Corn gluten meal is a very good suggestion, but, as Jellydonut points out, be careful because it will inhibit any seeds from germinating. New grass (brand new seeding) or wildflowers will be stopped, too.
Corn Gluten Meal is a great product if the weed is not already up. It should be put down twice a year every year (February and September) top prevent weeds. If the weed are up you should use an over the top. One that I recommend is Image it is not organic but not as harmful as something like Atrazean. The advice about fertilization is also very good a healthily lawn will strangle out weeds. I recommend Milorgranite or Ladybug (in Austin) as a good Organic fert.
That's why you lost grass to your using Roundup...it probably never really went dormant.
Your best bet is going to be a Fall pre-emergent...next Fall.
For now, you are boned...water and fertilize your yard for Spring and just try a localized spot treatment of roundup....don't go overboard.
You both are right while clover is not sign of lack of nitorgen, it can be a symptom. Weeds grow better when they don't have to fight with healthy grass and grass that lacks in nitrogen will not be as healthy therefore allowing weeds to grow. It is very important after treating for weeds to streghten the lawn.