No

AM2 vs AM3 has to do with Bios and Chipset not just socket.

Does this mean that I can not change the BIOS to do this then?

Thanks

You cannot change your Chevy to a Ferrari by putting Hitest Gas in it.

There is no such thing as AM2 to AM3 socket upgrade.

The only reason I ask is that we have 2 of these with faulty processors and it is said that AM3 is backwards compatable and I have some very cheap versions. But Dell motherboards are difficult to change the BIOS to compensate. So it looks like ebay to find a good AM2 to replace the faulty ones.

Specs of the 740.

www.dell.com/.../optix_740_en_new.pdf

web.amd.com/.../opti_740techspecs.pdf

web.amd.com/.../740_customer_brochure.pdf

Typical CPU

AMD Athlon 64 x2 2.0GHz 3800+ 2 x 512KB L2 Cache Socket AM2 65W

ADO3800IAA5CU

The Optiplex comes in three models.

Mini-tower (Model #DCSM),

Desktop (Model #DCNE) and

Small-form Factor (Model #DCCY).

Each has a different size power supply/Motherboard and only the mini-tower will support a 125w processor.

The "YP696" motherboard is also an "enhanced" version
which has 8MB of VRAM and can support up to a Phenom X4 9850
whereas the non-"enhanced" boards top out with 64x2 Brisbanes.

So it depends on a LOT of things as to what works or not.

Updating the bios is not optional.

look around for an AM2+ 95w Phenom x4 processor like the AMD Phenom X4 9750 2.4GHz
p/n: HD9750WCJ4BGH. 

This Thread is really old I know but I got a "old" Optiplex 740 with AMD Athlon 64 3500+ as Desktop Form Factor version. Also I have a 0PY127 Motherboard in my OptiPlex, so I can buy a 64x2 Brisbane and it should work or what is the highest CPU? Which  low-profile GPU can / should I use in the Optiplex 740?