ok so here is my input ( i am a student and am always after doing things on the cheap where possible, this has caused me to learn skills that i didnt previously have)
the truth be told you could build an entire computer system up for under £30 without problems ( including a crt monitor) if you stick to old parts and less fassionable brands if you stick to 'off the shelf parts' and dont go micro like a lot of people on here suggest
what you have to look at is what is your MAIN priority is it keeping the price down, keeping small or indeed having many gadgets?
i have an mp3 playing system that works ( boots within 1min) and plays mp3s happily it runs of all standard pc parts that cost me a total of £15 in fact it boots xp! admitadly it doesnt have a screen but i have a tft screen that i have lying arround ( so if i ever get the urge it is there)
ok, ok so i have to have mains to run it ( not ideal) but it cost me a LOT less than ppl here look at!!!
tho it is bigger than a number that you see ( has a simple but effective home made case)
if u r looking at quality/size go with fancy psu's and stuff but b4 you do look into cheap or free parts ( a good place to start is a local university, they throw out all sorts of stuff usually!!!)
hope i have been of some use