Available processors taken from:
http://www.crash-bg.com/CATALOG/computers/hp_xw9400.pdf
------------------------------------------------------------------------------------------------------------
One or two Dual-Core
1
AMD Opteron™ 2000 series
2
processors 2220 (2.80 GHz) and 2222 (3.00 GHz) with AMD64 Technology
2
, 1 MB of L2
cache, and 1 GHz AMD HyperTransport™ technology
3
; one or two Quad-Core
1
AMD Opteron™ 2000 series
2
processors 2376 (2.3 GHz), 2378
(2.4 GHz), 2380 (2.5 GHz), 2384 (2.7 GHz), 2387 (2.8 GHz), 2389 (2.9 GHz), and 2393SE (3.1 GHz) with 1 GHz AMD HyperTransport™ bus 6
MB L3 cache; one or two Six-Core
1
AMD Opteron™ 2000 series
2
processors 2427 (2.2 GHz), 2431 (2.4 GHz), and 2435 (2.6 GHz) with 2.4 GHz
AMD HyperTransport™ bus 6 MB L3 cache.
if bios reports lower clock values then may be this link can help you:
http://www.techsupportforum.com/forums/f15/solved-upgrading-xw9400-mobo-567269.html