TempFS is a memory based file holder which will only allow you to use half of whatever physical memory is available. If you have to store just short of 1GB of data, you'll need 2GB of physical ram available. It'll have instant access as it sits on memory. It's also dynamically changeable size wise.
Ramdisks however is a file with a file system inside of it loaded in to memory. It's difficult to dynamically change it's size and it's fast for reading but not writing to.
If you have to use one, TempFS is the way to go, or Sata 10k RPM raptor HDD would probably suffice