Seagate's SSHD (Solid State Hybrid Drive) combines a traditional hard drive with 8 GB of SSD. The SSD caches frequently accessed data. Whatever you use the most, gets faster and faster. This all works in the background, so works great with older operating systems that aren't SSD friendly such as DOS, Windows 98 or Windows XP.
Using SeaTools you can set any capacity, turning the drive into a 8, 32, 120 capacity drive for example. You do this by entering the number of sectors into SeaTools. One sector has 512 bytes, so a bit of Maths is needed, but you can just use the spreadsheet below.
Using SATA to IDE adapters you can use this drive in older machines. I had it working in a socket 7 Pentium machine and anything newer including Pentium II and III.
The drive also features SmartAlign technology, which deals with partitions that aren't aligned with the new 4K sector format. Under Windows XP I would still align the partitions though, it's very easy to do.