Those numbers are the PIO modes. ATA-100 is PIO Mode 4 and ATA-133 is PIO Mode 5. These modes have been effectively overcome by DMA (direct memory addressing) which is the mode used by most sytems now.
What is much more important for you is the drive rotation speed. This is the real determining factor in how fast your data will transfer to and from the disk. The best drives have a rotation speed of 7200 rpm, the next level is 5400 rpm and the slowest are 4200 rpm. Bigger cache is better than smaller cache, but the difference is not really significant for most purposes.
Generally, higher capacity drives are cheaper per gigabyte than smaller drives, but how large a drive you get depends on your requirements.