You can never go wrong with the DRZ400sm. Its not the best track bike because its alittle heavy and the suspension is soft, but its still alot of fun. The KTM's are a great bike. Alittle more high maintenance but still a great bike.
I just picked up a 2007 HUsqvarna SM610 last week. So far the bike is a blast. I'm still breaking it in so I can't give you a complete review on the bike yet. This is my first tard. I had the same questions you have about suggestions on a beginner tard before I bought it. I did a lot of research before I decided on which bike to buy and trust me, there is a lot of info out there. Do a search here, Speedzilla and ThumperTalk.
I would strongly consider what you intend on doing with the bike, how much maintenance you are willing to do, how much you want to spend and the availability of spare parts.
Jekyll is right. The DRZ400SM is a great bike. It's cheap, low maitenance, tons of parts out there and easy to work on. My buddy has one and he loves it. I wanted a little more power and was willing to do a little more maintenance, so I went with the SM610. The other bikes I was considering were the DRZ400SM, 2008 KTM 690SMC, Aprilia SXV5.5 and Husqvarna 510R. I hope this helps.