Mobility Radeon X1400 Performance

I'm not much of a gaming person but I did decide to purchase Halo 2 for PC a few days ago. I seem to be having some issues with gameplay choppiness (mainly about a half to one second delay when performing an action). I can get it running fairly smoothly when I put it into low detail level and low resolution (800x600) however anything above this really affects gameplay.

Here's my system specs:
Dell Inspiron 9400
Intel T7200 dual core 2 Ghz
2 GB RAM (667 Mhz)
ATI Mobility Radeon X1400 256 MB Hypermemory

Is there anything I can do to boost game performance on higher settings? Thanks.

I'm not really a gamer myself, but on looking at your specs, would think that your machine could handle it perfectly well. Is the sound ok ? Is it just the visual gameplay that lags ?

Thanks for your reply. The sound works just fine, it is just the gameplay that lags. I've found that using ATI Tray Tools to adjust the settings to Max Speed (just settings, not overclocking) and 4 GB of ReadyBoost slightly speed up the gameplay but unfortunately not enough to go to a higher detail level or resolution.

What was the requirements for the game ? I know that some games have compatibility issues with some video cards but think this should tell you on the back of the game box - have a look, does it include your card ?