IMHO you should be able to go with
That would be 62.5% of RAM with a sufficient amount of RAM for the Server OS plus memory for DB Connections
@kvisle recommended using mysqltuner.pl. That script is excellent for judging the amount of RAM to dedicate to join_buffer_size, sort_buffer_size, read_buffer_size, and read_rnd_buffer_size. Those 4 buffers added together are multiplied by max_connections. That answer is added to static buffers (innodb_buffer_pool_size + key_buffer_size). The combined sums are reported. If that combined sum exceeds 80% of RAM, that is when you must lower those buffer sizes. mysqltuner.pl will be very helpful in this regard.
Since all you data is InnoDB, you can make key_buffer_size (key cache buffer for MyISAM indexes) very low (I recommend 64M).
Here is a post I made in the DBA StackExchange to calculate a recommended size of innodb_buffer_pool_size.
UPDATE 2011-10-15 19:55 EDT
If you know you will have 5GB of data, then my first recommendation was OK. However, I forgot to add one thing:
The log file size must be 25% of the InnoDB Buffer Pool
UPDATE 2011-10-16 13:36 EDT
The 25% rule is based strictly on using two log files. While it is possible to use multiple innodb log files, two usually works best.
Others have expressed using 25%
However, in all fairness, someone from the original InnoBase Oy company expressed not using the 25% rule because of having larger InnoDB Buffers Pool.
Naturally, the 25% rule cannot work when having huge amounts of RAM. In fact, the largest innodb_log_file_size allowed using only 2 log files is 2047M, since the combined size of the log file must be less than 4G (4096M)
CASE IN POINT : One of my employer's clients has a DB server with 192GB RAM. There is no way to have 48G log files. I simlpy use the max file size for an innodb log file, 2047M. @Kvisle's comment on my answer simply gives a link stating you do not have to limit yourself to two log files. If you have N log files, they cannot total 4G. My 25% rule is just in a perfect world (DB Server with 8GB or less).