I got more RAM in an old laptop and it runs great now. £40 versus £400 for a new laptop? The processor isn't too quick but it runs app software quicker (well it seems to).
Quite often laptops came with a 4200 rpm disk drive, putting a 5400 rpm drive (which normally have more/better internal cache as well) can make an enormous improvement.
Should be able to pick up a 2.5 inch 5400 rpm fairly cheap