Hi stone576,
You may check the list of processors we have tested with Optiplex 755 in the previous post. Alternatively, you may find a list of compatible processors from the Intel website. A link for the same is shared in the previous post. Your system is shipped with a Intel Q35 chipset that supports maximum RAM of 8 GB.
32-bit Windows Operating Systems allow maximum usage upto 4 GB RAM. 64-bit Windows allows the computer to overcome that limitation; recognizes RAM beyond 4GB. If you upgrade operating system to 64 bit, you might be able to upgrade memory to 8 GB.
In reference to operating system, Dell has tested Optiplex 755 only with Windows XP and Vista. Drivers for same are available on support website. In case you are able to install Windows 7 64-bit on your computer, you may face challenges in finding the drivers for Windows 7 64 bit. As an alternate, you may try to install drivers for Vista 64 bit (might work) or may try to install in compatibility mode for Vista. If that does not work, you may need to check device manufacturer's website for the Windows 7 64-bit drivers (If available).
Please reply for further clarifications.