The difference between ATA100 and ATA133 is that ATA100 has a burst speed of 100MB/s, whereas ATA133 has the limitation of 133MHz. These are not sustained transfer rates, which hover around 40MB/s. Both ATA100 and 133 require a 40-pin, 80-wire IDE cable. In real world use, it means approximately jack squat. Maxtor is the only company that has gone ahead with ATA133 capable drives. This is really more of a marketing strategy than anything else.
SATA (SATA 1, specifically) has a maximum burst rate of 150MB/s. It uses a 6-pin data cable and a special power cable (something like 9- or 10-pin, and you can buy 4-pin molex connector to SATA power cable adapter for pretty cheap). SATA II has a limitation of 300MB/s, but this is a burst rate, not sustained.
If he has SATA ports on his motherboard, you should be set, except for maybe the SATA cable (they come with the retail SATA HDD kit) and a SATA power adapter, unless his power supply has one. If he doesn't have SATA ports on his motherboard, you'll have to get a SATA adapter card, which can run anywhere from around $25-50 depending on what and where you get it.
The drive itself should be good, especially considering it has a 5 year warranty from Seagate, which is a reputable company. $115 for a 200GB SATA drive with 8MB cache is pretty good on all counts. It's cheap, it's fairly fast (7200RPM is more or less standard these days), and it's more of a future-proof'd drive. IDE will still be around, but Intel only supports 1 IDE port on their newest chipsets, and we're starting to see SATA optical drives now.
One thing you need to think about with any drive over 137GB is that Win2k and WinXP need SP4 and SP1, respectively, to acknowledge the extra space.
