Also own a 990 (960 w/duplexer). Ive been refilling and it has been working out pretty good. You might want to consider
refilling your existing cart's, its alot cheaper but can be a pain, depends on your technical ability.
The worst that can happen is you ruin a cartridge and waste some ink.
HP's have the print heads built into the cartridges so no worries (ie Epson) about damage to the printer itself.
You can usually get 4 refills out of a cartridge before it goes bad, but some people get up to 8+ refills. It depends on the kit
you use. I bought my kit through a Canadian company but I hear this place
ink refills offers a better kit that uses a method that extends the life of your cartridges.
At least most of the people in comp.periphs.printers think so (not associated with company, just hear its HP kits are superior).
Cost is around $8 per refill.