There are two elements to Metal Gear Solid games ...
It is a niche game that has a very loyal following, which just happens to be the same type of gamer who becomes a videogame reviewer or hangs out on web forums; much like Okami, No More Heroes, and ICO but on a larger scale. These gamers are (probably) willing to buy a PS3 to play Metal Gear Solid 4 but are also very likely to have already bought a PS3 to play Metal Gear Solid 4.
The other side of Metal Gear Solid's fans are just general gamers who are interested in the game because it is a high profile game on a system they already own; these gamers are not very likely to buy a PS3 simply to play Metal Gear Solid 4, but if they had bought a PS3 (or were going to buy one anyways) they would probably buy Metal Gear Solid 4.
With that said, the boost for the PS3 on the release of Metal Gear Solid 4 could be similar in size to the boost the PS3 received from the release of Grand Theft Auto 4; but it will not be similar in size to the combined boost the PS3 and XBox 360 received from the release of Grand Theft Auto 4.
