whats the best processor replacement I can use for OptiPlex GX520

The Optiplex GX520 Mini tower has two PCI slots and one PCIe x1 slot. The small form factor and ultra small form factor versions have no PCIe slots at all.

Thus the only GX520 that has any hope of doing any decent gaming at all is the mini tower, but PCIe x1 cards are limited to the low end.

You could get creative by carefully cutting out the end of the PCIe slot to plug in an x16 card to run as a x1 but the old CPU and being limited to a max of 4 gig PC2-3200 makes that not worth it.

Yes, despite what Dell says, a GX520 can take 4 gig, if you get two of the right 2 gig sticks.

With a PCI ATSC tuner card and a PCIe x1 GPU with HDMI a GX520 could be turned into a cheap DVR.

If you can find one real cheap, a GX620 board will drop right in and you'll get 4 DIMM slots (still limited to 4 gig), a PCIe x16 slot and two more SATA ports so you can swap out the IDE optical drive for a SATA one - heck, go for two, or install another SATA drive in the floppy bay.

Going even crazier, if you want to do some metal work, other similar size BTX motherboards may be made to fit. You'll have to cut out the I/O panel area because it's a permanent part of the Matrix "Smith" case. (All three of the GX cases have names taken from The Matrix movies.) You'll also have to hack the front panel power, LEDs and USB ports.

Fortunately, unlike some *cough*HP*cough* OEMs using BTX, the Optiplex GX series used bog standard ATX power supplies so up-powering is no problem.

The GX520 will happily support up to a Pentium D 820 at 2.8 Ghz but note that you must have A11 BIOS and it will still give you a message at boot time saying it's unsupported. It still works, however.

$ lscpu

Architecture: x86_64

CPU op-mode(s): 32-bit, 64-bit

Byte Order: Little Endian

CPU(s): 2

On-line CPU(s) list: 0,1

Thread(s) per core: 1

Core(s) per socket: 2

Socket(s): 1

NUMA node(s): 1

Vendor ID: GenuineIntel

CPU family: 15

Model: 4

Model name: Intel(R) Pentium(R) D CPU 2.80GHz

Stepping: 7

CPU MHz: 2793.103

BogoMIPS: 5586.20

L1d cache: 16K

L2 cache: 1024K

NUMA node0 CPU(s): 0,1

Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx lm constant_tsc pebs bts nopl pni dtes64 monitor ds_cpl cid cx16 xtpr lahf_lm