bios or software OC - which is best

which is best, overclocking in the bios or with software like ntune?

the reason i ask is - does bios overclocking that runs 24/7 reduce the life of cpus and gpus?

will software OCs that work on profiles work? for example; if i use ntune with profiles, the system will run at stock speeds until i pick an application that has a ntune profile. like call of duty 4. Will software OCs NOT reduce hardware life?

I am building a new budget gamer with: asus p5n-d 750i, xfx nvidia 8800gt 512m, pentium D 925(i will replace this later with 45nm), 4G patriot 800mhz DDR2, 250G seagate sata150 hd.

If it is not obvious, i am a OC noob. I have OC'd my ATI 3870 but that is it. I have never OC'd a cpu. I dont want to fry anything.

Overclocking is overclocking. Mild overclocking with proper cooling won't hurt anything. Big overclocking may eventually lead to hardware failure, but how long do most people run hardware before they buy new?
Unless you crank something up and fry it instantly, overclocking won't reduce the life of hardware past what most people feel is a usefull lifespan anyway.
Watch your temps, everything in moderation, and raise things a little at a time is the key.
Software overclocking the CPU very rarely works very well.
Overclock the CPU using the BIOS.
I wouldn't even attempt overclocking that Pentium D, as it is already going to run hot at stock speeds. Any increase will likely be unstable.
Save your overclocking attempts for the 45nm CPU you see in the future.