At $30, a collection of four Dreamcast games on a disc seems slightly underwhelming -- even if one of them is Space Channel 5 Part 2. At $20, however, the Dreamcast Collection becomes infinitely more desirable. That's the same price you'd normally pay for Sonic Adventure and Crazy Taxi on XBLA (though the XBLA Crazy Taxi is on sale right now for $5!), with two extra games thrown in.
If ten bucks was all you needed to turn your opinion around, you can take advantage of this deal at Amazon. The site is offering the Xbox version of Dreamcast Collection for $19.99 as the Deal of the Day.