x64 bit cpu for old dell optiplex gx280

There is no point to X64 cpu in this model.  The GX620 is the same and has a 3.5 gig ram limit based on a chipset limitation not on 64 bit OS or not. You have a Hyper thread single core P4.

Zero advantage to EMT64 cpu on this model.

Intel didn't implement 64 bit until the 600 series on socket 775

Pentium 4 511 seems to have come in 2 flavors LGA775 and PPGA478

The GX280 is too old.  The GX620 can go all the way to windows 10 but it too cannot take advantage of more than 3.50 gigs of ram regardless of the physical amount installed.

well i would try to go for the gx620 but i am really poor so even if it is not worth it do you know of any?

 Intel Pentium 4 Processor 511 1M Cache 2.80A GHz 533 MHz FSB Socket 478 SL7E2

The first step would be to carefully remove your cpu to find out what S Spec it has and whether or not its LGA775 or PPGA478.

However I do not know if the GX280 all models even supports this CPU due to the Front side bus being 533Mhz and this part being Hyperthread.

http://ark.intel.com/products/27453/Intel-Pentium-4-Processor-511-1M-Cache-2_80A-GHz-533-MHz-FSB

PLGA775, PPGA478 (OLD GX280)

The MID Tower is MR Smith and likely has LGA775 socket instead of PPGA478 as the other Non Mr Smith GX280's have.

Depends on which socket your GX280 has.

The 280 was transitioned to use the same case in the mini tower as the GX620.

So there are GX620 and GX280's that have the same outside appearance MR SMITH Mini Tower Case.

There is no one size fits all answer because of the various TYPES of GX280.

http://ark.intel.com/products/27453/Intel-Pentium-4-Processor-511-1M-Cache-2_80A-GHz-533-MHz-FSB