Having experimented with refilled generic cartridges from places like Cartridge World I concluded that:
a) They didn't last nearly as long as the HP originals

The colour ones were crap and never worked at all - which the guy at colour world actually admitted and replaced it with an HP cartridge.
So I'm back on HP originals.
If you want to buy a new printer it is worth checking out Which magazine. They periodically reviews printers and always assesses what the costs per page of printing work out at. There are big variations between printers, even from the same manufacturer.