You need to wipe the HDD involved in the servers with products like Boot & Nuke
http://www.dban.org/
or Kill Disc
http://www.killdisk.com/downloadfree.htm
The Boot & Nuke option is the better of the 2 but if there are RAID or SATA Drives involved Kill Disc is easier to use.
You should never give away a working system or at the very least a Working System with a Used HDD. We junk the HDD's in any systems that we give away and if necessary fit new unused ones tot he case so that the unit can be used.
In the past I have seen cases where Data has been recovered from a old HDD on disposed hardware and from the Rulings from the court cases the new owner is the proper owner of that Data they they recovered. Not very good for a business who can and most likely will be sued for not showing [b]Due Diligence[/b] in securing their Data.
Disposing of old unused hardware from Secure Environments is always hard and depending on what procedures are involved it may prove impossible. But with the push to recycle things just throwing this stuff in a dumpster isn't acceptable either. However provided that there are no HDD's int he units there is very little chance of Data being let out of the business.
In the past I have fitted new HDD's and given the install Disc's to some charities when I have disposed of obsolete Hardware. HDD are cheap these days and there is not a massive expense involved with Donations of this kind. But if you have to load and setup the servers that is a different story. ;)
Col