You have to check with a Realtor in your area. According to your question, you'd be converting a 6-bedroom home into a 5-bedroom home. If that's the case, then--in most areas--the negative impact would be minimal. You'd run a much greater risk of a drop in value if you converted a 3 bedroom into a 2 bedroom, for instance. But 6 to 5 . . . shouldn't be a big deal.
However--big however--you say there are 2 in the finished basement. Are they true, legal bedrooms? With closets and legal exits? Or are they just relatively large rooms that you're calling bedrooms? If they're true, legal bedrooms, then my previous answer stands. But if you're talking about converting a 4-bedroom home into a 3-bedroom one . . . more risk of a loss in value. In many areas of the country, you'd still be OK, but that sort of situation is when you really have to consult with someone who knows property values--and what buyers are looking for--in your area.
4 homes sold in Zip 20736 in December 2008 for an average price of $541,113 down 4.18% from $564,734 average of 4 homes sold in December 2007. The averages don't mean a lot when you only have 4 to compare. Two of the sales were for less than $300,000 and two were for over $700,000.
You can look at market statistics yourself at the MRIS website. go to www.mris.com, hover on the "News" link then navigate down to "Market Statistics."