It can have an impact but the biggest question remains "who is your audience?".
If you're targeting US-based users, you'll want to make sure you're listed in Google Local and Yahoo! Local listings as well. Short of changing servers to one that's based in the US, just make sure you do everything you can to indicate that your business is located in the US and your customers will find you.
My host is based on the West coast in Vancouver, the server is situated in the US and I target Canadian traffic almost exclusively. In my case, the server location has had little if any impact at all.
By last count, I rank very well in my niche, at least I do for searches performed North of the 49th parallel!
This thread might shed some more light on your question:
Physical Location Of Server & Search Results
Here's a pretty interesting article that might also be of interest:
Google's dirty little secret - Netlash