The highest-clocked CPU I've ever seen installed in that model of server is the Intel X5470 (3.33GHz). The firmware or motherboard may not allow for the CPU that you're trying to use. Check the firmware release notes for more information. The X5482 (3.20GHz) is a 1600MHz front side bus CPU. The G5 ProLiants use the Intel 5000P chipset, which only supports (up-to) 1333MHz bus speeds.
From the ProLiant DL360 G5 quickspecs.
Intel® 5000P chipset supporting up to a 1333MHz Front Side Bus (FSB)
Digging a bit further, the chipset on that server, the Intel 5000P, does not support the FSB speed needed for the X5482 CPUs you have. Later versions, like the 5400 Seaburg chipset, do support that particular CPU.
Personally, I would try to find a deal on the X5450 (3.0GHz), X5460 (3.16GHz) or X5470 (3.33GHz) CPU kits (HP part# 487511-B21) if you're determined to keep using those systems. However, the advent of the Nehalem and Westmere CPUs in the ProLiant G6 and G7 range represented a massive jump in performance for the applications I deal with. You may realize a more dramatic speed increase by going that direction over the clock-speed improvement from upgrading 2008-era processors.