To find out what kind of controller you have now, check with your motherboard mfg. and look in the bios. If it shows DMA 2, then it is ATA33. Also, watch your screen during boot-up, it should list the drive mode.
An ATA100 hard drive will work fine with an older system. Your motherboard will most likely use ATA33 which will force your hard drive to follow. I would not worry too much about this however, even the fastest hard drives will only have to slow down a small amount when used on a ATA33 controller. ATA100 is not very useful at this point in time.
As far as which hard drive to get, check in the Hot Deals section and go for the best bargain. All the modern hard drives are quite good and you can't go wrong. IBM drives have a good reputation these days for being fast and quiet but I would just look for the best deal. Lastly, go for a 7200rpm drive only if you are looking to upgrade your system sometime soon. Considering your present system, 5400 will be fine. Good luck.