If you are looking to play actual Steampunk, then you could do a lot worse that OGL Steampunk. Every time I've run it, I've had a blast. It comes with rough ideas about a default setting, but mostly just gives you the tools to finish your own. The rules, however, are alll there.
If you want to "keep things D&D", then I recomend Dragonmech. It keeps a ton of the elements that make D&D feel like D&D but adds steam-powered mechs (along with clockworks, magic mechs, etc...).
Sorcery & Steam also "keeps things D&D" but also hints at a more Victorian setting. I stole bits from this book for my own Steampunk setting, which is a D&D world thrust headlong into the Steam Age before it was ready. S&S has a lot of good info on how magic affects a society and a magical society at that.
I ran Iron Kingdoms for 10 months, weekly, but ended up dropping it. I felt that it changed too much about D&D and it did so by dropping stuff instead of adding cool new stuff. That's not to say it doesn't add anything new or anything cool, I just didn't like the mix.
That being said, the Monsternomicon is the best monster book ever. Ever.
Good luck with your game!
-Tom
