unfortunatly I don't think it's really possibly to buy an upgradable system and save money, I mean to upgrade you generally buy say the best motherboard, and save something on the processor. so you can upgrade it later...
What I suggest you do is buy a fairly good system, and don't worry about upgrading it just yet...
For a mother board/processor combination I'd suggest anything from PCchips (as they are cheap) in socket a (because AMD chips are a lot cheaper than intel, certanily in terms of speed you get a whole load more bang per buck)...
I suggest that if you have a choice between a board with onboasrd sound and network etc you get that rather than one without, simply because it'll save you having to buy a seperate card...
the same goes for video, it's be beter to spend an extra tenner for onboard video than an extra 50 buynig a seperate card...
for Ram I'd suggest that this is a corner you can cut a little, 512 is ideal but 256MB will be adequate especially snice this computer is mainlyfor school and internet surfing...
the case is another areas where you can shave a few extra quid from the total...
but a very cheap case, even if you don't like the look of it, it'll be hidding under a desk anyway...
save some money by buying a small CRT monitor, rather than somethin nicer like a large flatscreen monitor...
in fact Ebay might even be the bet place to pick up a monitor...
get cheap headphones rather than nice speakers,
Are you UK or US?
lurks have provided some god US links, but if you need links to UK shops just ask...