Man, you can fill up 250GB in no time at all, as soon as you start downloading tv shows it gets crazy.
I originally got two 300GB drives for my PC (which already had an 80GB and a 120GB). I stupidly set them up in a Raid configuration so that I could effectively have a single drive with half a terrabyte of space, vain and pointless I know. The raid config never worked, kept erroring and I started to lose data. The only way to get all the stuff off though was to buy another 300GB drive. I bought one with the intentions of selling one of the older 300GBs.It never happened. Now two of the 300GB drives are 95% full and the other's on its way.
Mind you if you 'only' have 250GB you do tend to be a bit more space saving. I've got two or three backups of my 80GB drive stored somewhere on there which I really should sort through and delete. They'd have been dealt with months ago if I didn't have as much space.
Also, its not going to be *that* hard to install a 500GB drive if you're mildly technically minded. All you need are the right screwdrivers, a bit of patience, a few cups to sort screws into and a pen and paper to write down in which order you've taken everything apart. That way you really can't go wrong.