Unless you are interested in 3-way SLI or HybridPower (something that no LGA775 motherboards support at the moment) get the 8800GTS. You can overclock the card yourself, and although you might not be able to match that GTX's 760MHz you should be able to at least come close.
Edit: Whoa, that is an outrageously high price on that GTX. Screw that.
homerdog, I think you missed the OP saying those prices are Danish. Anyway, for the difference in price between the 2 cards, no matter the currency, the 9800GTX is definitely not worth it. Go with the 8800GTS 512.