As @MartinSmith said if you ever upgrade to SQL 2008 then a filtered index would be the perfect solution. However in the mean time as a general case ANY added index will increase your load time. Small indexes less so than large ones.
One thing I would look at is if you have an existing index that can be modified. Assuming your existing queries are using a given index then adding the bit column to the end of that index should have minimal effect on inserts and the positive effect you are looking on your queries.
Next thing to look at is "Do I have a lot of indexes already?" There is no hard and fast rule as to what "a lot" is but I usually go by a rule of 10 indexes is the limit unless I REALLY need a new one.
Last thought, test it out on a test instance. Set up a table with a few million rows, run your load on it, add your index then run your load again and see if you notice an significant increase in load time.
Only you can really decide what "significant" is. I have machines where adding 5 minutes to the load time is "significant" and others where I could safely see a couple of hour increase.
Another option is to partition your table. You may have to use a partitioned view if you are not using Enterprise edition but even so it should help. You put your bit 0s in one partition and your bit 1s in another. Assuming you are only inserting one version or the other then you may even speed up your inserts.