"Same for me. Only obvious drawback I can see as opposed to an external USB card is sound quality..?"
Simple just get yourself a decent interface. BTW thats not as easy as it sounds, do your research...
"Also, noticed that a fair few people have commented that the traditional rectangular "bedroom" can not reproduce sound perfectly. So how effective are bass traps really? I have a fairly untrained ear, never having had monitors before, so I question how much I'd even notice the difference..?"
I will let someone else answer...
Pretty much everything above answered by 3za.
firstly, the thing about only having a headphone jack is you don't have an interface for any speakers. so either get yourself a USB or Firewire interface for monitors or if you're just getting hi fi speakers get your self an amp and you can use that as an 'interface', and just jack to jack it to your pc, its what i used to do with my hifi speakers.
but yh, an interface is the most important bit in my opinion, there are so many out there you just need to work out what you want, what you want to spend and what you actually need. I have a small Audio Kontrol 1 for home which is a nice small portable interface that i use just for home production. but obviously your needs may be different, can't stress the RESEARCH bit enough.
finally onto the 'sound reproduction' of rectangle rooms. and bass traps.
the effectivness of bass traps in general comes down to many different things, such as, type of bass trap, shape of room, positioning, type/placement/responce/levels of monitors etc.
also, it's not about the room 'reproducing' the sound, thats down to the speakers, (i would go into more detail but im tired and it's too late for me to bother, plus i dont think you'd get it if i told you, no offence

), it's about what happens to the sound once it's been produced by the speakers. obviously (or you should know) sound interacts with whatever gets in the way of 'the path of sound', be it your walls, you bed, wardrobe, or whatever else. this causes absorbtion of certain frequencies of the sound(freqs absorbed is dependant on surroundings) and reflections of the sound and, to put it simply, too many of these reflections can give your ears an untrue representation of the sound.
most problematic to an untrained ear is where the bass can become 'boomy' because too many reflections take place and the sound can become 'trapped' and the bass frequencies can sound ridiculously loud. for example if i open and stand behind my door in my room, the bass behind the door sounds stupidly loud, but go and sit in front of the monitors and it sounds perfect. this is where bass traps become usefull, they are not always nesecary, i dont use one in my room, but for example where you try and mix you do get this 'boomy' bass, you might need to make use of one and allow more of the bass freqs to be absorbed to stop this from happening.
sorry if this is too simple, but its late and im very very tired, and anyone please feel free to elaborate on this or add/take away stuff.
basically what im trying to say is, it's all about where YOU are, theres no formula to put on it, or no one who can tell you exactly what you need, it's all about experience and testing and doing it for yourself. just dont feel like you NEED all these bass traps and acoustic treatment when you dont even know how to choose good monitors, just get some cheap monitors and your production will sky rocket as it is, worry about all this stuff when you have mre experience and atleast know what youre looking for, and im not trying to be mean or whatever and i may be wrong, im just trying to help you out based on what i get from your posts. anything else, feel free to message me at a more reasonable time of day :)time for some

i think...
