I've read almost everywhere that Sennheiser PX100 are VERY good for their cost, I can find them i Italy at about 30€ (more or less $25) and it's a very interesting price, they've got also a very good frequency range (15-27000 Hz); can anybody tell me more about them? Are they good for an mp3-cd portable player?

And also... how much is important to have a good player, (I ask you beacause I've seen a very cheap one, that also read MP3s, at 29€ (Euroline CD181), and I seriously doubt about its quality...any opinion about it? Thanks

If you could get them this cheap, I would say go for it. Both their construction and sonical quality are quite good. They are a bit on the dark side with slightly boosted and bloated bass though. You could also consider getting Koss PortaPro which has very similar character (however suffers from the same problem with bass) with slightly more detail but costs more.
If you want some decent isolation from the outside noise, you should also try PX200. They have IMHO more neutral presentation than the above ones. But their proper positioning on your ears is critical. If you don't get the fit, then the bass is gone. They have also a bit recessed highs.
And don't forget that the PX100 and PX200 comes equipped with a nice storage case!
Please take this insight with a reserve - it's very subjective and the best thing you could do is to try to find a place where you could test these phones for yourself with your player.

P.S.: Forget the frequency range, it's more a marketing tool than useful information. It really says nothing about the quality of the phone.
About the quality of the player: yes, it's important too, but the greatest impact on the sound quality have the headphones.

Just want to throw in my €0.02.
By recomendation in Yet-another-WhatShouldIBuy thread on HA
I chose the px100, and am so far very pleased with them.
They are cheap, light, very comfortable to wear, good sound (and a awesome sound:value ratio imo) and can be easily 'folded' to fit a pocket. Heh, it even has a nifty case for it.

The only thing that is annoying to me is the short cable. When plugging it to my computer at work , I have take them off if I want to move/turn more than 20cm from my usual computer position (which is very often ).

I bought the PX-100 about 7 months ago. It still works damn well. The sound quality is amazing-considering the size and price. If you are getting them that cheap, go for them. They are probably the best headphones for your DAP.

Bought them too, and must say the same said, very good for the price, comfortable, and comes with a good case (it is logic, I do a portable use, and I wouldn't like to put them somewhere in my pockets when I stop listening music in the street or everywhere, specially for their price, as them are not a cheap aiwa headphones)

I recommend them.

I use it since several months.

Very good. Go for it...

I don't know that one. One difference between players is the amount of hiss or noise. They may have a signal/noise ratio, which is closely related, but that is the rating at the highest volume the player can put out. If you have sensitive headphones and/or aren't deaf yet from too loud music, then you will have the volume turned down somewhat, but the noise level is often a constantt... it the s/n ratio goes down and it is more noticeable.

That said, I have an Iriver player that has a slight noticeable hiss, a Sony with an annoying hiss, and a Neuros that I hear no hiss through phones (there is some on the FM broadcast mode)

Beyond that there are differences in user interface, how easy to navigate and load music. Trying one is best.

For many brands there are dedicated email forums on Yahoo which you read to see what people who own them have to complain about (or cheer) after they've had a chance for the sales hype to leak out of their brains.