Linux will cache as much disk IO in memory as it can. This is what the cache and buffer memory stats are. It'll probably do a better job than you will at storing the right things.
However, if you insist in storing your data in memory, you can create a ram drive using either tmpfs or ramfs. The difference is that ramfs will allocate all the memory you ask for, were as tmpfs will only use the memory that your block device is using. My memory is a little rusty, but you should be able to do:
# mount -t ramfs ram /mnt/ram or
# mount -t tmpfs tmp /mnt/tmpand then copy your data to the directory. Obviously, when you turn the machine off or unmount that partition, your data will be lost.
