The 745 is not a XEON based system and is not suitable as an ESX server.

VT-x is basic for most dells as early as GX620 but is not supported in all processors.

Pentium D 960 for example is required for Pentium D series.

is that processor work with OptiPlex 745 ?

Pentium D 960 does work with the Optiplex 745 but not all models.  Its a 130W cpu that requires copper heatsink and 305W power supply which means Mini tower only.

J9761    Heatsink and Shroud Assembly; PERFORMANCE, MT
M8802    Power Supply, 305 Watt, PFC, MT
M8805    Power Supply, 305 Watt, PFC, MT
M8806    Power Supply, 305 Watt, PFC, MT

The Recommended CPU for 745 VT is E6000 Series.

http://www.dell.com/downloads/global/products/optix/en/opti_745techspecs.pdf

Intel® Core™ 2 Duo 1066MHz FSB Socket T with Dual Core technology  XD, EM64T, 2MB and up to 4MB L2 cache, EIST and VT (E6000 series) which is Intel® Virtualization Technology (VT-x)  it is NOT VT-d and it is not suitable as an ESX server.

I would recommend

http://ark.intel.com/products/27249/Intel-Core2-Duo-Processor-E6400-2M-Cache-2_13-GHz-1066-MHz-FSB