Estimating hot data size

All,

I'm trying to estimate the size of my hot data set, and wanted to get some
validation that I'm doing this correctly.

Basically, I'm using the sum(heap_blks_read + idx_blks_read) from
pg_statio_all_tables, and diffing the numbers over a period of time (1 hour
at least). Is this a fair estimate? The reason for doing this is we are
looking at new server hardware, and I want to try and get enough ram on the
machine to keep the hot data in memory plus provide room for growth.

Thanks,

Chris

Example:

*Time*

*Total Blocks*

2011-02-16 11:25:34.621874-05

123,260,464,427.00

2011-02-16 12:25:46.486719-05

123,325,880,943.00

To get the hot data for this hour (in KB), I'm taking:

(123,325,880,943.00 - 123,260,464,427.00)* 8 = 523,332,128KB

Correct?