Yes, the 533Mhz RAM will be your biggest obstacle. Without access to a memory speed selector, every Mhz you push the FSB beyond 266Mhz will also overclock your RAM by the same amount. I predict you'll be able to get 50Mhz tops out of that RAM.
Getting a different motherboard won't help you at all. You need either access to memory speed selection (ie: RAM multiplier) or faster RAM.
Edit: By the way, MemSet allows to change RAM latencies from windows. It's possible there's a tool that will let you change the RAM multiplier. Google is your friend.