It’s a big day in video games.

Take-Two Interactive said worldwide sales of Grand Theft Auto IV have surpassed $500 million in its first week on the market. More than 6 million games have been sold at a retail value of at least $60 a copy. And id Software announced “Doom 4.”

On its opening day, GTA IV sold 3.6 million units with a retail value of $310 million. Strauss Zelnick, chairman of Take-Two, said in a statement that the game made entertainment history, surpassing the first-day sales of any music title or movie or game.

Microsoft claimed the same thing with the launch of “Halo 3” last year, a game that sold more than $300 million worth in its first week and $170 million in the U.S. alone in its first week. But that game was notably available only on the Xbox 360, while GTA IV is selling on the Xbox 360 and the Sony PlayStation 3.

From a business view, such sales are astounding and they suggest that the hardcore market for gamers is healthy, regardless of the shift toward casual titles on the PC and the rise of the Nintendo Wii, which appeals to broader audiences beyond young male gamers. GTA IV represents the high tide of the crowd that loves to kick back, defy authority, and play extremely violent crime games. The resiliency of this franchise is amazing, given the U.S. and other parts of the world are stuck in a recession.

That’s great news for Take-Two and its Rockstar division that launched the game. It might force Electronic Arts, which has made a hostile bid for Take-Two, to rethink its $26 a share, or $2 billion offer. Analysts have said that GTA IV sales are baked into the current Take-Two stock price (TTWO: $26.26 today) ; but on the other hand, some analysts have increased their estimates for Take-Two in the past week or so based on higher expectations for GTA IV sales.

Meanwhile, id Software of Mesquite, Texas, announced that it is working on “Doom 4,” the latest series in another high-octane billion-dollar franchise in the video game industry. Judging from past hits, the next Doom game should push the bleeding edge graphics to a new level and present a test as well. Gamers are getting increasingly picky about their games. GTA IV has upped the ante for expectations of a great game.

The designers at id, led by graphics guru John Carmack, are going to have to step up themselves now. It’s a never-ending cycle of one-upping the other guy. I can tell that they’re going to try. That’s because id also noted in its press release announcing Doom 4 that they have a lot of jobs open for skilled developers. These games aren’t getting any cheaper to make and id has here-to-date operated with teams that are much smaller than the average for the industry.

It’s not that hard to see why GTA IV is beating expectations. Sometimes big games can’t live up to the hype. But GTA IV has scored a surprising number of perfect reviews, probably more than any other video game in history, with an average Metacritics score (an aggregation of review scores) of 99 out of 100.

I’ve been playing GTA IV at a casual pace, given my heavy blogging duties. I’m only about 5 percent of the way through the game’s missions (it has a stat page that tells progress). So far, I like it but mainly for the story, not for the graphics. I expected the graphics to blow me away. In some static scenes, it does. I like how the environment changes from day to night, the glowing of nightclub signs, and how real the gritty streets look.

But there are plenty of times where I look at a scene and think, “That looks fake.” When I saw a preview of the game, I only saw what wowed me. Now, naturally, familiarity is sinking in and I’m noticing the things that annoy me. I can, for instance, see some blocky straight lines where there should be smooth curves in the characters. There is also a significant amount of mismatch between the lip movements and sounds of the characters. I went to a dart-throwing bar and it took me a few minutes to find the door that allowed me to get inside the building. I expected that to be better.

But the story is engaging enough to keep me going. My character, an Eastern European immigrant named Niko Bellic, doesn’t even have a weapon yet since it’s so early in the game. But, playing as Niko, I’ve managed to accidentally drive over a lot of pedestrians in my mad rush to be on time for a date with a girl named Michelle or to make an appointment with a loan shark boss. The cops came after me just once, even though I carjacked a few cars and ran over a ton of streetlights. Hence, the realism of the game is a notch below what I expected as well. My driving skills are improving though and I’m behaving better in front of the police now.

I’m not ready to pass judgment on this game yet. But it’s got me playing continuously. And that’s probably true for the rest of the country as well. It will be interesting to see what happens when Nintendo strikes back with the ultimate peaceful game, Wii Fit, on May 19. That title, already a hit in Japan, lets players use a “Wii Balance Board” to weigh themselves and play various fitness games on the Wii console.

I expect the result will be that top flight titles will continue to set new records, but the mediocre titles that launch in the near future are going to suffer. GTA IV is going to have a pretty big blast radius. I’ll probably be playing this one for a while. And when Doom 4 comes out ( no launch date yet), it better be a step above GTA IV.