The general consensus around here seems to be:
F-Zero GX will be good but it wont sell systems.
Mario Kart will be awesome and it will sell systems, appealing to the mainstream.
I would have to PARTIALLY disagree with both of those statements. First of all, it looks to be that F-Zero will be the better of the two games. Mario Kart Double Dash!! has so far the least racing depth of any Mario Kart yet (apart from the two people in a vehice), especially since there are no power slides and no coins to collect. The game is less challenging and slower than previous Mario Kart games. It will still be great, but there is a reason why F-Zero GX turned heads at E3. First of all, the game has amazing graphics, and stellar special effects. Nothing quite beats racing against 29 other vehicles at the same time. The game is extremely fast, challenging, very solid in single player and multiplayer modes, features great courses, it's full of racing depth (with turbo and on track boosts, several different attacks, being able to pass not just left or right but also above and below, and vehicles that vary greatly in playing style and abilities) and has great controls.
As for being able to sell systems, Mario Kart will likely be able to do no more in that regards than Mario Sunshine (This is of course just my opinion). Obviously most Nintendo fans see the game for what it is, but like most everything else, that's not the case with the mainstream. The game's image to the mainstream infortunately may no longer be that of a cool kart racing game, but that of a colourful racer with a pink princess that "only appeals to kiddies". F-Zero on the other hand has all the tools and potential to be a system seller, but likely won't be marketed enough. It's fast, a cool image and appeal, it has great graphics, and features "cool" characters like Captain Falcon. However, if it's not marketed to death, then you guys are right and it likely wont sell systems.
More on topic, here's a release list for Nintendo and Capcom games. If all the big hits released near the same time, it could have the potential to give the Cube an adrenaline boost, but as they aren't, it'll probably be the same old story, with some of these games being high on software sales lists.
Nintendo
Mario Golf: Toadstool Tour - July 28th
F-Zero GX - August 25th
Pokemon Colosseum - September 20th
1080: Avalanche - September 29th
Kirby Air Ride - October 13th
Pikmin 2 - October 27th
Final Fantasy: Crystal Chronicles - November 3rd
Mario Kart: Double Dash - November 18th
Mario Party 5 - December 14th
Capcom
P.N.03 - September 9th
Resident Evil: Code Veronica - September
Viewtiful Joe - September 30th
Gotcha Force - November
Disney Hide N' Seek - December 16th