Rotational frequency has become a large selling point (5400/7200/10000/15000RPM) recently, so you shouldn’t have too much difficulty finding out what the rotational frequency of your HDD is when you go to buy it. Now of course the faster it spins, the faster it can read from the disk (there are other advantages also), but be aware that the faster it spins, the more wear is put on the drive, and the more likely it is to encounter errors and malfunctions later in life. Hard drives are pretty much the only systems within your computer that have moving parts (CD-ROM, floppy disk drives, and fans also have moving parts, but they’re all cheap and if they break you won’t lose all the data you’ve accumulated over the course of owning your computer, be sure to back up your data regularly!).
If you know anything about physics or engineering, then you know that moving parts produce friction and wear, and a faster spinning platter (the part that holds the data and spins) means more friction, and therefore more wear on your drive. Of course the faster it spins the faster it can read data from the platter. So what can you do? well again you backup your data as much as possible and you get the fastest spinning drive money can buy (which is 7200RPM for ATA devices (the sort you are buying) and 15000RPM for SCSI and some Serial ATA devices (you might get Serial ATA, that depends on your motherboard)). So look for a “7200RPM” label on the drive you’re thinking of getting, it’s important!
The “buffer” I’m referring to is the transfer buffer that temporarily stores data to read from or write to the hard drive. The reason for this is that the bus (data channel between the hard drive and the motherboard) can only handle so much information at a time, and it takes a long time for the data to be found and retrieved from a platter (random seek time), so it takes a big chunk of the requested data and stores it in a very fast data buffer to be sent at the bus’s convenience. So with a larger buffer you can queue more data for transfer over the bus without having to keep sending requests to slowly find more data from the platters. Some newer drives come with as much as 8MB of buffer capacity, but it is more common to see 2 or 4MB. Usually a drive with a large amount will use that as a selling point and print it on the box/website, otherwise you can assume it has a 2/4MB buffer. My advice is to find one with as much as you can afford.
Now for the transfer rates you’re going to need to do some searching, the best way to find these numbers is to go to the manufacturer’s website and look at specification sheet for the model you’re looking to buy (actually this is the best way to get all the information I’ve talked about so far). One thing to look out for is that you may read something that says 133MB/s or ATA/133 or SATA150 or 150MB/s, and these are not the numbers you are looking for, those numbers describe the bus connecting the hard drive to the motherboard, and are theoretical peak numbers, which tell us virtually nothing. Also, if you are buying a Maxtor, don’t bother looking up actual transfer rates, as Maxtor is apparently afraid to release such information. Not to say Maxtor’s are bad drives, actually Maxtor has risen to the top in terms of technical support and they are much more willing to replace broken drives than some other companies, which is important since some ridiculously large amount (something like 10 or 15%) of hard drives are either dead on arrival or die within the first few weeks of use. As with the last two points, when looking at transfer speeds (if you can find them) always try to pick the one with the highest numbers you can afford.
Notice that the first three of the 4 major aspects were all speed related? That was done intentionally to give you a sense of the importance of speed in the hard drive, because they are very very slow creatures, and for your gaming experience you don’t need an enormous hard drive so much as a fast one. To increase speed substantially more you can set up a RAID, which is a network of hard drives which (in RAID0 mode) share portions of the same data, such that accessing data takes 1/n time (n = number of hard drives attached) compared to 1 hard drive. To do this you really need a hardware RAID chipset, which are commonly included on higher-end mainboards.
Finally, capacity is an unnecessary nicety. The smallest hard drives you can buy today that are still within those higher speed classes are all about 30GB, which is more than enough for your games. But since games are not everything, and if you happen to be an iTunes member or like to download anime then you will really want some extra space. So I would recommend getting something between 60 and 120GB, any more and you’re going to be paying through your teeth, any less and you might be looking to buy a supplementary hard drive in a couple years. And, oh yeah, 300GB hard drives from Maxtor are out, but they are comparatively slow, so I wouldn’t recommend getting one for gaming, I like to stick to single platter drives (40-100GB usually), as they produce less heat and are less likely to break down on me.