How do I monitor disk IO in linux?

Running Redhat Linux 5 on my test system.
Disk IO on sda is described by this oid:
.1.3.6.1.4.1.2021.13.15.1.1.4.17 (writes in bytes)
.1.3.6.1.4.1.2021.13.15.1.1.3.17 (reads in bytes)

When I get the value by hand (snmpwalk) it comes back as 234949468431 in bytes. / 1024 / 1024 turns it into MB, about 234949MB, or 235GB.

So I tried ((.1.3.6.1.4.1.2021.13.15.1.1.4.17/1024)/1024) and got 0 back. Then I tested against .1.3.6.1.4.1.2021.13.15.1.1.4.17, and got 0 back, from OpManager. They I tried putting this value into a simple java program. Only to get this error "integer number too large". I am guessing that OpManager can not handle number bigger then the definition of an integer, about 64000?

Is this a problem with OpManager, or am I doing this wrong?
-Adam