I've gotten my NF7-S R2 based system up to 220MHz FSB before I got file system corruption. I'm happy at 200MHz FSB actually, my 2500+ is happy as a 3200+ and my system is really fast there.
As far as mods go, you could add a passive cooler to the SB chip, clearer audio with no crackling is a possible benefit to that. You could also put a big, passive cooler on the NB chip (the zalman model is popular and I use one successfully) to reduce system noisiness. Aside from those, there is very little need to mod the NF7-S. It does well as it is for most people.