I wouldn't go north of 7k. Above that you could risk getting valve float from the stock valve springs (main reason not to) and you start getting into strange vibration issues due from the stock rotating assembly not being perfectly balanced. Whats the point though. Unless you have modifications to support it, you won't make power north of it anyway.
For higher rpm you would want a few things. A new tach, for one, if you have the factory tach. Then once you start reaching over 7500 you'd need a different valve-train like titanium valves and higher lift cams. The lower weight of titanium valves would allow you to rev higher without having massive spring rates- increasing hp and reducing damage to the valve guides. That's an expensive proposition though. You might be better off power wise to making more power lower, but that won't increase your MM top speed where rpm is needed. From what I've read, the stock D20 crank is good for 8k rpm, but if you're going for the max- steel might be required for much more reaching.