I'm surprised that no one has posted this yet. I can't be the only one who has it. I picked up my copy last night at GameZone...then proceeded to play until 2AM.
I'm sure it's no surprise that this game is awesome, so I won't waste any time with that. There are more cars than anyone could possibly need. Probably the best improvements are the tracks...i've already run over 20 races and haven't seen the same track twice. There are TONS of tracks. World Circuits, City Courses, Mission Courses, Scenic Courses...there's a drag strip (Las Vegas), and a testing ground with a slalom course and skidpad. They have Nurburgring, Infineon, Talladega, Firebird, Sears Point, Motegi...some of my favorites. Also the physics engine is much improved. Yes it's more realistic...but it also allows for more fun...powerslides are very easy to induce and very realistically controllable, and when you hit another car, both cars react exactly as they should. No more brick-wall-like opponents.
I can say for sure that there is no SVX...sorry. There also is no Trans Am
Lastly, but probably most importantly, you can import your licenses and up to $100,000 from your GT3 memery card. So no more struggling along the beginner courses in a Daihatsu.
I highly recommend this game to anyone who loved GT3.
Go get it.