a gt440 with gddr5 memory could be considered slightly better. That one has gddr3, so no, it's worse.
I just found this
That's one of the best deals i've seen on uk/Europe sites.
The 6670 is a rebranded 5670 with gddr3 memory. The 5670 is discontinued but you can still find some here and there.
That one has a full 1gb of gddr5 and will perform better than any of the others linked. If you don't mind ebay it's your best bet.
edit: also, is yours a 305W or 255W psu? that optiplex seems to come either way.