HP XW4600 motherboard upgrade guide
You can use any P45 motherboard that matches your CPU (just search for a compatible list online)
It isn't required to be P45. Any 775 that fits your CPU is acceptable.
Check if your situation qualifies as ATX.
It seems better to opt for a microATX case, as they are more widely available.
No, you're not limited by the connector.
Yes, there is a proprietary connector which would require serious modding. What is your budget. You may just want to upgrade