I am running a Optiplex 755 w/ a Core 2 duo E2140 CPU. What CPU can be purchased for more power in the 32 bit range? Can a 64 bit CPU still run with the mother board and software?
32 bit or 64 bit is Windows, not hardware, you can install Win7 32 bit or Win7 64 bit on any hardware configuration, 64 bit is much better, because it can address more RAM, 32 bit is limited to 4 GB (actual about 3.25 GB or so).
Now about CPU upgrade.
This is the best manual for Optiplex 755 http://www.dell.com/downloads/global/products/optix/en/...
If you can not view it, download and open.
Maximum upgrade is Q6600 and Q6700, where Q6600 is easier to find and cheaper.
You must have Copper heatsink in order to upgrade, look at picture below to correctly identify yours
If you have Aluminum, you still can change it to Copper. If you live in USA it is very easy to find, if you live elsewhere, finding right parts is more difficult, so where do you live geographically?
Also, please identify your form factor from picture below, as it is necessary for other upgrades