Don't waste your time and money on seven-years PC. You will be hard to find LGA755 CPU worth the hassles of upgrading. It is not just the CPU, everything else is two generations (at least) behind.
Not sure what you're asking. Are you asking if it's possible? Intel Q45 and Intel Q43 support Core 2 Quads. Be careful though, they don't support Core 2 Extreme, so don't buy one of those.
Hello! You'll be fine upgrading your cpu to a q9400 (Look on Amazon, they're around 50$) and if you want to upgrade anything else in you're optiplex, check out my video and hopefully it can help with any other questions https://www.youtube.com/watch?v=HwQpm-mimEU