How old is this server? (specs) you might want to stay with a 2.4.x kernel.
You certainly don't want to stay with FC1 though... unless you want to run an insecure server (or it's on a closed network) or you want to patch all security updates manually.
The difference in system "load" between 2.4 and 2.6 is not that big too; often, 2.6 even outperforms 2.4.
Klaasjan
