Above 80FPS (Note, above 80FPS. It may be a slightly higher limit, but 80FPS is the maximum I was able to consistently get to work, using RivaTuner because it has a harder cap. A cap of 77FPS may be needed to go online), the game will actually send its request to “login” the server too quickly, and you’ll be stuck in a state in which you’re half-spawned, no weapons, and don’t have a presence to the other players. You’re basically a ghost, because the UE2 developers forgot that they could make a timed delay, instead of frame-based delay. So, it’s the server host that misses the login command.
So, essentially nothing when you’re on your own. It’s an online multiplayer only issue.
That said, UE2 supports high FPS surprisingly well, with that as its one and only issue, besides countless games relying on DX8, like Star Wars Republic Commando, notably. I actually learned of the issue from Republic Commando, which has a somewhat modified form of the UT2004 multiplayer. It’s honestly not very different internally, I’ve peeked at it out of curiosity. That game is a masterpiece of UE2 technology, with how many different effects they added to the game. Even better, the game has a fix to the bumpmapping. It’s pretty nifty.