OptiPlex 745 Small Form Factor CPU upgrade

The power supply is too small to support those.

You are falling prey to the MHZ Myth.  Netburst was abandoned because cranking up the MHz with 20 pipelines or more DOES NOT make it faster.  This is why everything now uses the CORE architecture  in the Core 2 Duo Series.  2.66 Ghz Dual core cpu is MUCH Faster than 3.8 GHz netburst Pentium 4.

If you want more mileage out of your system Install a Pentium D 900 series like a 915 or a Core 2 Duo series up to 2.66 Ghz.

UY273 Presler P4 Kit, 915, 2.8G, 2X2MB, 800FSB, C1

Intel® Core™ 2 Duo 1066MHz FSB Socket T (E6000 series)

Intel® Core™2 Duo Processor E6700
(4M Cache, 2.66 GHz, 1066 MHz FSB) is max.

Ram continues to run at 533mhz regardless of what the CPU FSB is.

This is a limitation of the chipset and DDR2.

The S Spec is the numbers written on the top of the cpu.

Intel® Core™2 Duo Processor E6600

(4M Cache, 2.40 GHz, 1066 MHz FSB)

with SPEC Code(s)

SL9S8, SL9TZ, SL9ZL

Intel Core 2 Duo 6600/2.40GHZ/4M/1066/08 SL9S8