In order to replace the config from tftp-server in to RAM you will need two commands for this to work. Here is an example.
Lets say you have a tftp server on the inside and its IP address is 135.108.123.55 and you would like to configure the PIX so it always has knowledge of that server and the filename the config should be named.
tftp-server inside 135.108.123.55 /config
When performing a write net the running config will be pushed to the TFTP server and the filename will be called "config"
wr net
Building configuration...
TFTP write '/config' at 135.108.123.55 on interface 1
[OK]
Now lets say you would like to copy the file in TFTP server directly back into running config RAM. This can be done by typing in "configure net" from the prompt. The configure net command is dependent of the tftp-server command.
HTH
R/Yusuf