Check into Knockout roses. They are very disease resistant and flower for a long time. Another favorite of mine is the Cranesbill type of geranium. They are a perennial and bloom most if not all of the summer. (I'd stay away from the bee balm, it only blooms a few weeks and can get invasive.) One that doesn't necessarily bloom but has great colors is Heuchera or Coral Bells.
Search the plant names you get and you should find their bloom times and such. When you look for plants, pick the ones that give bloom time over 2 months because something that is Apr-May usually means maybe a week in April and maybe a week in May to the plant companies. :]
Also, you may want to get help someone at a local nursery. There are people that can advise you on site all the way to someone who will work up a garden plan for you to have. I've also noticed in this economy, they are more willing to work with someone on a tight budget and in a variety of ways - including free.