I don't think the HDD would cause much in game lag. Load times, yes, but the bottleneck is probably the processor.
"The joy of the PC world..."
Yes, you have to upgrade more frequently with PCs, but it's worth it if you like RTS games, and the game he's playing. You just can't do it on consoles.
But judging by his CPU and HDD, he has a 2-3 year old system, and he's upgraded the video, which I don't think is going to help much on RCT3 because it looks like an MMX game engine, meaning it generates video with the processor mostly. MMX games have a grainy 2D/3D look to them.
If he's a big PC gamer, he should consider building a new sytem within a year.