http://russia.visahq.com/

I've used them for every visa except my student one. They provide you with the invitation and get you the visa and even register you once you're in Russia. I've used them in both the US and the UK. It really doesn't get any more one stop shopping than that.

As for hotels, I'm sure you can find something online. in st.petersburg the Nevskiy group is pretty good. I've stayed in three of their hotels. They operate at least 5 right in the center of SPB and the prices, as far as I remember are pretty decent. Shouldn't be more than 100 bucks a night. Jan is a great time to go. Its very pretty in the winter.

You'll love it!