If you decide to buy new, hold on until you can get Windows 7 as the OS.
It's easy enough and not expensive to add a larger hard disk (up to 250GB, assuming IDE), especially if you clone the old to the new. Cheapo battery and new disk would keep you going for another year or so for around £120.
What do you mean by "further tech. malfunctions", btw? As long as the screen is OK and the case (and hinges, keyboard, connections etc.) are sound, then what's to break, apart from the hard disk?
SD