Some indexing advice for a Postgres newbie, please?

Hi folks,

I have a single-user application which is growing beyond the
fixed-format data files in which it currently holds its data, I need a
proper database as the backend. The front end is written using Lazarus
and FreePascal under Linux, should anyone feel that makes a
difference. The database will need to grow to around 250,000 records.

My problem is with the data field which is the (unique) key. It's
really a single 192-bit integer (it holds various bits of bitmapped
data) which I currently hold as six 32-bit integers, but can convert
if needed when transferring the data.

How would you advise that I hold this field in a Postgres database,
given the requirement for the whole thing to be a unique key? The
first 64 bits change relatively infrequently, the last 128 bits will
change with virtually every record. The last 128 bits will ALMOST be
unique in themselves, but not quite. :(

Thanks,

Brian.