Back on topic - "What a concept"
First of all, it really depends what you want. If ALL you want is Hydro Thunder, there are versions on the PC (actually really good, especially the graphics) and video game consoles... so you have plenty of different ways to pick it up. I'm not a huge fan of the game, but I know that both the DC and PC are fun and look great, as I have both versions.
And really, if you want the game, how much is too much to spend? Granted, the cheapasses in all of us want stuff cheap, cheap, cheap. But I traded the bulk of my Xbox games, several PS2 titles, the Grand Theft Auto Music Box set, and maybe even an N64 title or two when Steel Battalion came out just to get it. And while it's VERY cool, I've played the game MAYBE 5 hours total... including about 30-45 minutes on the online expansion to decide it just wasn't what I was hoping it would be. (Not fighting battles in similar mechs, and only having 4 to start was SERIOUSLY lame.) But I digress...
As to the Dreamcast itself, I was NOT an early adopter. I didn't import one, didn't think I'd EVER like Sonic Adventure, and really thought I was better off with a PS2. And while I do prefer my PS2 for the hugh selection of games, the Dreamcast DOES have better graphics (most of the time), and a decent selection of titles that are fun to play. As a wrestling and sports game fan, Royal Rumble, World Series Baseball, the NFL and NBA 2K series... all good stuff. And obviously there are a TON more. So I think if you get a Dreamcast, and your a "serious gamer" that would eventually look for more titles... you'd really enjoy it.
But, it's completely up to you. Personally, I got a black Sega Sports one and 4 black controllers eventually, replacing my white system and perpherials. And while it's in a box at the moment (I'm looking for a new place, HOPEFULLY my first house), I'm looking forward to getting out my DC, N64, and even my Sega CDX eventually to play some of the great games of my time. Great stuff...