Usage makes a difference. If you're main usage is gaming, hard drive speeds won't affect you much. You will see differences in level loading times, but hard drive speeds won't affect your fps. An SSD is a nice thing, but if you can't afford it, most standard 7200RPM HDDs will do the job.
Oh, you meant the Momentus XT. The standard Momentus is a regular HDD and not a hybrid, but the Momentus XT has the hybrid HDD/SSD.
No, the spinpoint is a standard magnetic HDD and will not perform as well as the XT for *some* tasks. However, I would be relatively certain that a standard drive and the XT will perform the same in gaming - even for things like load times. The SSD part of the XT essentially acts like a giant cache and then the driver/OS intelligently tries to store your most commonly used files on the 4GB SSD. However, 4GB is not very much and your operating system files are used far more often than your gaming files, so the OS files would be the ones that saw the speed increase. Because of that you wouldn't see any gaming difference between the XT and a traditional hard drive. However the XT would boot and navigate windows faster than traditional hard drives.