For the past 8 months or so 320GB drives have been at the best price point. First the IDE 3200JB and then the 3200KS from WD really brought down the price/gigabyte in the market, now the WD5000KS 500GB drive.
I think over the next few months we'll see 500GB to be the sweet spot for HDD value - the release of new 500GB-1TB drives from Maxtor, Hitatchi and Seagate should really bring the price down to the kind of territory that 250GB drives were this time last year.