you could put RHEL5.7 on a 620
BUT it will cost you
RHEL is not free there is a $299USD PER YEAR REQUIRED
Also the rhel5 series is a bit old .Is there a reason you can not use the current RHEL6.1 ?
so a $299 / year
OS on a $350 computer ( yes there is a cheaper one BUT the standard IS GOOD )
you are paying for the support contract ,so have you read the install instructions ?
or asked your support rep ?
-- the rhel 5 install guide --
If you do NOT intend on buying the REQUIRED RHN license then DO NOT use RHEL5
there are FREE rebuilds
"CentOS 6.1" and " Scientific linux 6.1 "
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search the red hat knolage base for that error
the solution IS in there
your RHN username and pass IS required for the search ( the one you set up with the contract )