So what I propose is this. We don't need specific benchmarks for the underclocked computers. What we need is a series of validations tests, or a series of regression tests, if you prefer.
This does sound reasonable. But there is a few things that comes into play here. Gaming and DVD playback is heavilly depending on both floating point calculations and the speed of your graphics card.
I run an underclocked XP2500+ to XP1500+ speed passively cooled, so I will do my best to answer your questions.
Time to boot is faster than a stock XP2500+. Ok, this really sounds too good to be true, so lets explain why. First of all I have higher FSB and lower multiplier, so memory and HDD speed is really faster than a stock XP2500+. Secondly I have stripped off excessive hardware, and this saves some time during boot. I normally run without floppy and without optical drive. In adition to this I did some tweaking in the OS and removed all crap at bootup, so in total this machine boots a lot faster at XP1500+ speed than my Intel 3.0 machine at work does.
The conslusion is that the CPU speed is not very important for boot speed, but FSB makes a little bit of difference.
In addition to this I run my machine 24/7, so I seldom reboot.
Starting a program is mostly disc-bound, and slightly memory bound, so CPU speed is not very important here. This goes for both starting IE and starting Acrobat reader. If you really want exceptional disc speed you should get an IDE flash drive (the 800MB ones are really cheap). This will probably make your programs start at least 5 times faster.
Can I play mp3 at high rate and browse at the same time? No problems at all. This uses very little CPU, so I guess even the VIA C3 will do quite fine here.
Can I play DVD's smoothly? Yes, but I typically don't do it on this machine with only integrated video on motherboard (Abit NF7-M). I don't have a 5.1 system connected, so I have not tried that.
The truth is that when I want to play games and play DVD's the speakers will make so much sound that I can tolerate the machine to make a little sound. So, I change change vcore, multiplier and FSB to clock the machine up to over XP2500+ levels. Maybe it is just vanity, but getting a few extra fps is always nice.
There is a major difference here with underclocked machine and a machine that is slow from the start. With a slow machine you can not increase speed when you need to. The VIA C3 is not made for gaming. If you are a gaming freak you should have one gaming rig and a VIA rig for more normal stuff. DVD playback is border-line for the VIA CPU. With a graphics card that does most of the work you can get great playback.