I too was (and still am) a distance runner first and foremost, but I suggest you visit your LBS and see if they have a trade in program in which perhaps a bike from a previous year came in. Chances are you can score a decent bike that was initially out of your range (for the current model year) for somewhere below your targeted budget. Perhaps something like a 2004-2005 Trek 1500 or even something a bit higher in their lineup from 2004 or 2003. Just make sure that the LBS looks over it all and will guarantee that it isn't defective in any way (and that it fits most importantly!). I think you could pick up a pretty nice road bike this way, and possibly still have some cash left to pick up a helmet, spare tubes, pump, waterbottles, pair of shorts, etc... You might even be spared the cost of water bottle cages and a pump if they were left on the old frame.
If you are strapped for cash, check out some stores online (Performancebike.com or Nashbar.com among others...) for the accessories, as many can be found at ridiculously cheap prices there. As much as I like to support the LBS, sometimes it just doesn't make fiscal sense to purchase from them.
The road biking will be a great supplement to your running and you will most likely enjoy it. Best of luck with both and hopefully you'll be getting out on the bike soon.