I never really experienced much slowdown in Halo. But I have heard many complain that it isn't too friendly even on high-end systems, so I wouldn't worry that it's your graphics card fault, probably just poor optimizing by the devs of Halo. You really won't notice much if any of a performance increase unless you play with AA and AF turned on. You'd get higher framerates, but because you shouldn't be dipping below 30fps now in any games now, you really won't notice any improvements. Stick with your 9700Pro for now, you probably won't even need to upgrade when HL2 and Doom3 come out because of the resolutions you play at aren't high enough that you'd benefit from a faster card.
As for Unreal, there shouldn't be any performance issues with your card, it most likely a software issue, try patching the game, or tweaking the settings because there's no reason to upgrade from a 9700Pro for Unreal.